Understanding the Dietary Needs of Fish: A Comprehensive Guide for Aquarists
1. Introduction to Fish Nutrition
General knowledge of fish nutrition is essential for success in aquaculture, be it at a garden pond or in a public aquarium. Unfortunately, fish are often on the receiving end of neglect in aquaculture, simply because of the lack of emphasis on nutrition. Acquiring knowledge of the basics of fish health and disease will provide tools to aid in the prevention and control of problems due to insufficient nutrition. Diets provided to fish should mirror the natural diets of the species present as closely as possible in nutritional content, particle size, and frequency. Properly designed and formulated diets will ensure optimal fish health and growth. This chapter outlines fish dietary information likely to benefit hobbyists as well as many professionals working with non-commercial aquarium species. Thus, it is written with an emphasis on species found in the ornamental trade and those kept within the home aquarium.
Dietary contents can be complex in structure and composition, further complicated by an array of feeding strategies adopted by varying species. This chapter is subdivided into six subsections, each addressing a key aspect of fish diets and nutritional requirements. First, a more in-depth overview of the three main macronutrients in fish diets (proteins, lipids, and carbohydrates) will be provided. Then, a discussion of minerals in fish nutrition and the important problem of calcium metabolism will be explored. Next, water in relation to fish dietary needs and health will be addressed. Regulatory chemicals, including hormones and digestive enzyme secretions, will also be discussed in light of dietary content and amount. Finally, dietary needs in relation to varying developmental stages from embryonic to post-larval animals are explored.
1.1. Importance of Proper Nutrition
One of the most crucial aspects of raising any animal, including fish, is making sure they consume a healthy, balanced diet. While some species can be fairly easy to feed, for example, bread is actually incredibly dangerous to the digestive system of most fish, while other species may contain critical nutrients as opposed to offering their health attributes. Diet can influence behavior, as well as lending a greater visual aesthetic and coloration to an animal that feeds dominantly on meats and foods high in carotenoids. Experiments on the feeding habits of animals like koi, it was determined the percentage of egg output was substantially higher for those females who had been fed a diet high in animal proteins as opposed to those who were fed a high and strictly vegetarian diet.
In the case of herbivorous fish, an imbalance of certain nutrients can considerably lead to, among other complications, digestive blockages. Fish are, in general, categorized into one of approximately three categorizations depending upon the precise biology of their feeding, known as omnivorous, herbivorous, and carnivorous. This is not particularly rigid and there are numerous species that can considerably eat from the different categories and have no problem with it. Despite this, it is commonly agreed upon that carotenoids, natural pigments often related to many fruits and vegetables, are linked to vibrancy of color.
2. Types of Fish Diets
In answering the question of what fish require in their diet, aquarists need to understand the types of food that most fish consume in the wild. These foods fall into three categories: carnivory (organisms regularly consume whole or half-organism prey), herbivory (organisms consume whole or sectioned plant material), and omnivory (organisms consume a mix of animal and plant materials). A descriptive list of the vitamins, micronutrient minerals, and macronutrient minerals required by the animals in an aquarist’s care as a part of their diet can be made by understanding these three main diet types.
Carnivorous diets are very high in proteins and lipids as compared to plant-eating foods, and in many cases, critical nutrients such as some types of fatty acids can only be found in animal tissues and body parts. Herbivores are different in species, lifestyle, ethical expectations, and existing body parts. The protein and lipid values in carnivore feed were very low; these foods required high fiber, while fewer nutrients are condensed per bite. Due to the fact that animal matter can contain many of the same nutrients as plant matter, lipid and protein values in omnivore diets are more physiological. Omnivores may be included with both herbivore and carnivore. Omnivore food focuses on animal foods, which can restrict diet variety in other ways such as hybrids or other fish. Omnivore animals cannot normally handle high fiber.
2.1. Carnivorous Fish Diets
Carnivorous fish are adapted to eat a predominantly protein and lipid-based diet. These are predators that prey on smaller animals. Carnivorous fish will usually use aerobic metabolism at a steady pace to swim and wait for opportunities to capture prey. The pectoral fins are the primary swimming fins. These are fish that require foods rich in protein and lipids. High-quality, complete diets from reputable manufacturers are generally safe and will meet a fish's nutritional requirements. This is especially true for captive populations, which are often much smaller than their wild counterparts.
Fish require several classes of nutrients, most of which they can synthesize from the basic components of their diets. However, there are a minimum of 41 nutrients (amino acids, fatty acids, vitamins, minerals) that fish cannot synthesize, and these nutrients need to be obtained from their diets. High-quality ornamental fish foods are designed to provide a complete and nutritionally balanced diet for most aquarium species. Fish food manufacturers are very good at formulating diets to meet these special nutrient requirements for aquarium fish. Additionally, a range of live, fresh, and frozen prey items can meet the dietary needs of carnivorous fish. Carnivorous fish in the wild consume a variety of prey. Fish and their relatives can consume entire vertebrates. Offering these larger prey items to fish in captive systems may be recommended to replicate their natural diet.
2.2. Herbivorous Fish Diets
The digestive systems of herbivorous fish, those that primarily consume vegetation, contain a number of unique physiological features that are linked to their feeding habits. In order to produce enough energy to feed, these species require a large amount of fiber in their diet to stimulate digestion. In general, these fish obtain vitamins from their food and require an abundant supply of essential vitamins in addition to high fiber. Providing herbivorous fish with a diet fit for their needs can lead to visible benefits.
One can tell, according to Janiak, if herbivorous fish are receiving enough essential nutrients if they are growing at the proper rates, produce enough offspring, and if they can survive in the long term. Deficiency can lead to diseases, death, or other complications during development. While fiber is not an essential component in a fish's diet, not all fish require it as a part of their diet. Herbivorous fish have long digestive tracts in comparison to other types of fish including other species of fish that consume vegetation and fruits. Herbivorous representatives have consorted hindgut fermentation as an adaptation to the cellulose in their food while still retaining a fully functional mid- and foregut. Minerals and essential vitamins are a major part of herbivorous dietary requirement analysis as these fish obtain their minerals mostly through their food intake. Containing an abundant amount of essential vitamins such as A, C, K1, and other vitamins are important when formulating a herbivorous diet. Containing excess vitamins can cause vitamins to leach from the food in the form of water-soluble vitamins. Laying off the choice of food containing any essential vitamins can prove to be detrimental to long-term captivity.
2.3. Omnivorous Fish Diets
As the name implies, omnivorous fish eat a more varied diet compared to carnivorous or herbivorous species. They need a combination of both animal and vegetable matter to thrive. The exact ratio of such food can vary between species. However, it is generally agreed that a balanced diet is preferable in and of itself. Many aquarists fail to provide a variety of high-quality foods, increasing the risk of malnutrition, leading to poor growth and a weak immune system. Other common health issues often associated with malnutrition include lopsided swimming and fatty liver disease. Additionally, some aquarists would prefer to avoid live or frozen prey items because of their associated risks or costs, making it important to discuss 'balanced' foods as a stand-alone topic.
In the wild, the diet of catfish can vary greatly from one population or species to another. That said, the stomach contents of a wild Corydoras catfish typically contain at least 50% insect larvae, while their intestines often have a relatively high content of pollen. The exact food species will not be available in an aquarium, but be sure to provide both anaerobic (cucumber, zucchini, cholla wood, etc.) and aerobic (stable granule and flake foods) plant matter. A diet of frozen Daphnia, New Life Spectrum small fish formula, and fresh peas, or something similar, will go a long way in meeting those protein and plant needs in a captive environment.
3. Nutritional Requirements for Different Fish Species
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Different fish species vary in terms of their nutritional requirements. While some studies have provided information on the nutritional requirements of fish species, some others have also noted the need for specific nutrient requirements of particular fish species. It is known that very little information is available on the quantification of the nutritional requirements of more than 50 fish species, especially for freshwater and saltwater fish species, in contrast to freshwater prawn aquaculture. There is no single feed or feeding strategy to accommodate the brain feeding capacity of the diverse finfish species. Hence, different fish species require customized feeds and feeding regimes that are suitable and cost-effective, and are also environmentally friendly. However, many feed producers use a diet or feed for all systems, which is only expected to sustain the life of the fish, without enhancing the commercial productivity of the fish.
3.1. Freshwater Fish
The range of nutritional requirements of freshwater fish is vast, and the above-mentioned points are meant to be established rules. The interaction of these (and other) nutrients, or more specifically, dietary factors, can show wide variability according to the fish species concerned. Therefore, selective feeding habits must be addressed before considering dietary influences imposed by some of these (or other) rules. The nutritional needs of most freshwater fishes appear to fall within the following parameters.
A daily intake of 20-62% protein is adequate for middle-weight, warm water (15-20 °C) dwelling species such as trout, Murray cod, catfish, barramundi, silver perch, and carp. Warm water, low-level species, particularly silver perch, are relatively sensitive to digestible protein (DP) intake, with a dietary protein content of 30-45% promoting the best growth. Containing at least one n-3 highly unsaturated fatty acid (HUFA). The HUFAs, in common with most other fats used by animals, are long-chain polyunsaturated fatty acids (PUFA), but with a peculiar arrangement of double bonds. The CP content that is widely reported (i.e., the CP by combustion) in a feed or ingredient may not have corresponding biological significance to dietary protein. The bioavailability of "dietary protein" (i.e., CP as a function of growth) after digestion largely depends on the following factors: the balance and amino acid availability of dietary proteins and amino acids; the presence of growth-limiting (2nd limiting) amino acids (e.g., lysine, methionine, cysteine, etc.); the presence of habitual physical, chemical, or biological contaminants that reduce feed palatability, limit digestion, or reduce the bioavailability of nutrients. CP is susceptible to a decline in bioavailability caused by overheating during feed manufacture (especially at inadequate inlets of equipment) and due to the chemical reactivity of bound components of food ingredients such as the Maillard reaction, oxidation, and lipid-protein cross-linking. Optimal dietary protein levels may be markedly influenced by factors related to animal health and genetics as well as environmental conditions (e.g., water temperature and quality).
3.2. Saltwater Fish
The 9 essential amino acids needed, both directly by the fish and as starting materials in the fish's own metabolic processes, are: arginine, histidine, isoleucine, leucine, lysine, methionine, phenylalanine, tryptophan, and valine. Fish turn to their muscle-based tissue storage of these amino acids for their metabolic needs if they aren't found in their supplied diet. Foods containing these amino acids are called complete proteins. Choline, for example, must be directly supplied in the diet so that free betaine can be produced to maintain osmotic balance in the cells and thus prevent dehydration. Omega-3 fats must be directly supplied because they are needed for the fish's vision. Astaxanthin is a potent natural antioxidant, with some studies indicating it may even have the ability to reduce cancer.
Salinity can affect the food source for your fishie's consumption; she needs LOTS of it in her dietary feed to survive. Most commercial aquarium foods for (H2O2) marine foodfish exceed their amino acid and fatty acid needs, as well as providing the essential (and some nonessential) micro-, macro-, trace- and major minerals; vitamins; color-enhancers (astaxanthin) and other additives for health and parasite disease prevention. Fish keepers who actually have the resources to purchase commercial marine fish food may choose to buy frozen fish food instead of freeze-dried or air-dry food.
4. Selecting the Right Fish Food
Fish food is incredibly diverse, with some options focusing on specific varieties, such as Tetrafauna ReptoMin Floating Food Sticks, which are intended for aquatic turtles. Others, such as Omega One Super Color Flakes, are formulated to meet the needs of species that have a wide diet, such as a betta, arawana, larger cichlids, goldfish, or oscars. Once you know the particular eating habits of the organisms in your aquarium, try to pick a food that fits their nutritional needs. Look for nutrition information on the package. However, just because total protein is good doesn't mean a food is ideal. Check the specific ingredients listed. Better fish foods list whole fish, shrimp, or squid. They tend to use the whole organism, which contains the most complete set of amino acids and nutrients.
Commercial foods should contain all specific nutrients and vitamins a fish needs to survive. It is not necessary to supplement standard commercial food items, although incorporating some variety into your fishes' diets can be healthy. Other than dry food, artificial fish flakes, dry dog or cat food, manufacturer canned food, or frozen food can be purchased at most local pet stores and larger chain stores. Pet stores can also order commercial frozen fish foods. Home-based fish foods can include live brine shrimp, earthworms, or smaller fish such as minnows or some type of fish fry, if fed in proportionate quantity, or the fish can be allowed to nibble on live plants. These are fun to provide now and then, but a fish's diet should mostly be food designed specifically for it.
Increasing commercial fish food diversity, stocking only one type of fish will ensure all fish can receive the correct nutrition. Different fish eat different things. Try to pick a fish food that contains the most food designed for the different kinds of fish in your aquarium. Quality is also very important in fish food. A professional cook does not use a cheese food product instead of cheese in quality dishes, so don't use a cheap fish food that lists soy flour or any product like it as the first or second ingredient. Even worse would be one that lists poultry or beef by-product as the first ingredient.
4.1. Commercial Fish Foods
Commercial fish foods are widely available in the market, offering a variety of forms and types. Each type is labeled as providing adequate nutrition for a specific list of fish species. It is important to note that fish do not need to eat the same thing every day, nor do they have to adhere strictly to a pellet diet. Like most organisms, it is beneficial for fish to consume a varied diet of whole foods.
The following section covers the four most popular commercial fish foods and provides an informal explanation of their contents:
- Fish food flakes: These flakes are commonly used in the aquarium trade. While they may not be the most nutrient-dense option, almost every fish will eat them, and most fish can survive on them for a considerable amount of time. The flakes primarily contain the ingredients listed on the front label, such as various whole fishes, shellfish, crustacea, fresh whole kelp, spirulina, fresh whole fish fillets, fresh whole shrimp, fresh garlic, and freshwater mysis shrimp. The ingredients are listed in order of quantity, with whole fish being the highest in quantity. Additionally, the flakes may contain other ingredients such as bakers yeast, Carp, Anchovy Oil, Vitamin and Mineral Pre-Mix, Ascorbic Acid, hyperparin tips, choline, niacin, kelp meal, salmon meal, vitamin E, porphyra, carotene, chlorophyll A, d-activated animal sterol (Vitamin D), riboflavin (Vitamin B2), niacin, pantothenic acid, folacin, Thiamin (Vitamin B1), pyroxidine (Vitamin B6), and biotin.
5. Conclusion and Recommendations
In conclusion, it is important to understand the dietary needs of fish are not only based on species-specific adaptations but also on their ecosystem. Aquarists have a lot more knowledge about taxonomy and systematics of fish than we do about what they eat. There are reasons for these circumstances: often the microscopic animals and plants, and broken down organic matter that forms the diet for detritivores are not visible. Many of these diet items are not commonly seen in fish stomachs, making them difficult to identify. Understanding fish diets can be frustrating to those who want to know more or newcomers in the field but I hope this guide provides a basis from which to build on and incorporate new findings and techniques.
It is clear from all of the discussion in this guide that a simple diet or just flake food is not providing the right nutrition to our fish. It is important to know a little bit about what fish eat in the wild; regardless of whether you collect fish as ornamentals or food or keep tropical fish in a tank. A quality diet should mimic the natural diet as much as possible. A balanced diet that contains all the essential nutrients for your fish is also important. Proteins are the most expensive ingredient in the food and these are often overlooked when keeping native fish in the aquarium. Most native fish are ambush predators, eating food when it is available in their environment. Provide hides and shelters in the environment keeping other stress indicators in mind, aggression, water quality etc. (Cottrell et al.2020)(Xu et al.2020)(Roques et al.2020)(Aragão et al.2022)(McClements & Grossmann2021)(Sarker et al.2020)(Ebm et al., 2021)
References:
Cottrell, R.S., Blanchard, J.L., Halpern, B.S., Metian, M. and Froehlich, H.E., 2020. Global adoption of novel aquaculture feeds could substantially reduce forage fish demand by 2030. Nature Food, 1(5), pp.301-308. [HTML]
Xu, H., Turchini, G.M., Francis, D.S., Liang, M., Mock, T.S., Rombenso, A. and Ai, Q., 2020. Are fish what they eat? A fatty acid’s perspective. Progress in lipid research, 80, p.101064. [HTML]
Roques, S., Deborde, C., Richard, N., Skiba‐Cassy, S., Moing, A. and Fauconneau, B., 2020. Metabolomics and fish nutrition: a review in the context of sustainable feed development. Reviews in Aquaculture, 12(1), pp.261-282. wiley.com
Aragão, C., Gonçalves, A.T., Costas, B., Azeredo, R., Xavier, M.J. and Engrola, S., 2022. Alternative proteins for fish diets: Implications beyond growth. Animals, 12(9), p.1211. mdpi.com
McClements, D.J. and Grossmann, L., 2021. The science of plant‐based foods: Constructing next‐generation meat, fish, milk, and egg analogs. Comprehensive Reviews in Food Science and Food Safety, 20(4), pp.4049-4100. academia.edu
Sarker, P.K., Kapuscinski, A.R., McKuin, B., Fitzgerald, D.S., Nash, H.M. and Greenwood, C., 2020. Microalgae-blend tilapia feed eliminates fishmeal and fish oil, improves growth, and is cost viable. Scientific Reports, 10(1), p.19328. nature.com
Ebm, N., Guo, F., Brett, M. T., Bunn, S. E., & Kainz, M. J., 2021. Polyunsaturated fatty acids in fish tissues more closely resemble algal than terrestrial diet sources. Hydrobiologia. springer.com