Feeding and Nutrition in fish.

There have been quite a few posts recently about fish nutrition and many of these have been slightly misguided in their understanding of how fish nutrition works. So as I work as a fish scientist I thought I would help clarify some basics about nutrition and feeding in fish.

There are two types of diet when talking about feeding in fish, live food (living or frozen) and prepared diets (pellets, flakes ect).

Now the aim of feeding fish is fundamentally to provide all the necessary nutrients for the fish to survive, grow and reproduce. To determine what this is we look towards nature, what does the fish eat in the wild and how can we bring this into the aquarium? This is where live foods come in, these in theory are what the fish would be eating in the wild. (There are limitations to this argument of course, but for the sake of argument, this is as detailed as this will be for the moment). So, the live foods are our idea of a “perfect” diet, what the fish would chose to consume in the wild and provide all the nutrients which are required. Thus, this has led to the belief that live foods are the best for our fish. But due to the limitations brushed upon above this is not necessarily the case.

Now the idea of prepared diets is to emulate or improve upon the wild diet ( live food). This has been done in aquaculture to such a degree that live foods are now confined to the use of inducing first feeding (larvae need live foods to start feeding in most cases) and the transition to prepared diets is done as soon as possible. However, this is based upon almost 50 years of research into what a handful of species require, costing Trillions of pounds of resources and effort. In the aquarium, we are still a long way off producing diets which can cater for the ideal needs of all our aquarium fish. However, they are still able to provide the key nutritional components for a wide range of species.

So, when we are feeding our fish we can look towards practices within aquaculture to improve our hobby, our practices and the welfare of our fish. Aquaculture, you say, this has no bearing on what I do in my small 125L aquarium in my living room. And you would be partially right, aquaculture focuses on high growth rates, with low feed use and feed one species of fish in high numbers. Where we want slow growth rates, minimal waste, good colouration. However, there is a lot we can take from Aquaculture to explain nutrition and feeding in our own fish and make improvements in our hobby.

In the following thread I want to discuss several topics regards fish feeding and nutrition in Aquaculture, and how we can apply these to a home aquarium.

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Diet composition

The composition of any diet is key to providing the macronutrients for our fish’s growth. In the hobby we seem to focus on the ingredients list this is the wrong way of going about this. In Aquaculture the focus is not on the ingredients, but on the proximate composition of the diet. It’s protein, fat, minerals (ash) ect. This is the more important part of the package. For most fish species we keep the macronutrient requirements have been studied and using this we can match up what our fish need, to what the diet provides. Diets which are for specific groups of fish “cichlid” or “catfish” will have macronutrient profiles which fit many of the common species requirements.

Protein, in most diets protein is the largest section of the diet and can come from a variety of sources. Fish meal or Krill meal are the two most common ingredients which provide the bulk of the protein into the diet. However, this is also a highly unsustainable practice, and other protein sources can be used to replace these two. Animal proteins are commonly used to do this. These usually come in the form of bloodmeal, or animal meal (waste from the food industry) and can be included in the diet to reduce the demand for fishmeal. This is not a poorer quality of protein for the diet and most fish are able to digest this just as well as aquatic derived protein. Plant based protein these have been derived from high protein plants such as cassava, palm, and grains such as wheat and barley husks. These again can substitute and supplement the animal and fish protein in the diet, however some fish do have problems digesting plant proteins, and this is not a common inclusion in carnivorous fish diets.

Fatty Acids, are usually the next largest component of diets. This usually come from two or three sources. Fish/ Krill Oil, usually added into the diet to increase its fat content. These again can be quite unsustainable and there tends to be a replacement for other sources. Marine Algal oil this is very uncommon but is starting to make an inclusion into diets, as an aquatic derived oil, like fish oil it contains most of the Fatty Acid profiles which are ideal for aquatic species. Plant oil this is a common inclusion as a replacement for expensive fish oil, however plant oils tend to be higher in the wrong type of fatty acids and not the Omega 3’s which are required by fish.

Minerals, mineral components in fish diets and important for maintaining osmotic balance, bone formation, digestion, eyesight, embryo development and many more. This is quite tricky as mineral requirements are highly individual within species and high levels of certain minerals can have negative effects. Mineral composition in diets tend to come from all the other inclusions and are commonly supplemented by highly mineralised additives such as bone meal and feather meal.

Vitamins, as with minerals vitamins usually come from already included ingredients and supplementary additives, in this case usually come from a powdered form of the vitamin. Vitamin requirements are also highly species specific and it is hard to generalise requirements here. However many vitamins play important roles in growth, development and the immune system

Carbohydrates many herbivorous fish require carbohydrates within their diet, and this is commonly taken from aquatic algae or plant-based products. Many carnivorous species also benefit from inclusion of carbohydrates within diets which can improve the absorption of nutrients and increase gut transit time. Carbohydrates also act as binding agents within the diet, which help the pellet or flake maintain consistency in the aquarium. They also can give the pellet or flake it’s characteristics, such as floating, sinking ect.

This is obviously a very basic run down on the key ingredients of a fish's diet and there are plenty of caveats and exceptions, especially when you are dealing with hugely different taxonomic groups

Feeding rates, and feed conversion

A large portion of aquaculture nutrition discussion is regarding feeding rates and feed conversion ratios. That is to say, how much you feed VS how much fish you get out. In many aquaculture species this is very close to 1:1 with tilapia being the closest to a 1:1 FCR, with some of it’s life stages reaching a FCR of 0.7:1 (0.7kg of feed produces 1kg of fish). This is obviously not the focus of our feeding, but we can take lessons from this. To achieve these FCR’s there are strict tables to follow regarding feeding. As a rule of thumb most fish, to sustain a high growth rate get fed about 2-4% body weight per day and this is fed over the entire day period and is temperature dependant. Now this is massively too high for our needs, but again we can extrapolate.

This level is designed to do two things, reduce waste and provide the maximum growth rate and relies on a simple principal of fish biology. Let’s say we feed our fish 2% body weight per day of that 2%, 50% goes to energy use, 30% goes to growth and 20% is waste. If we up the feed to 4% per day now 50% is still used for energy, 20% now goes to growth and 30% goes to waste. Fish produce more waste by percentage, when fed more. If we now feed 1% body weight per day 50% goes on energy, 40% goes on growth and 10% goes to waste.

So, although this might seem counterproductive, feeding less, can improve the effectiveness of your diet. Feeding less also allows us to reduce the production of waste within our tanks and allow us to maintain better water quality. Overfeeding is a common issue among many hobbyists, especially those who are quite new. Realising that we may only need to feed a 10g fish 0.2g of food per day, or less will probably seem like such a tiny amount of food. But understanding this fact can lead to better practices and care of your fish. Realising that fish (ornamentals especially) do not need to feed every day also comes as quite a shock to many people (obviously there are exceptions). But reductions in feeding rates can have a significant reduction in many issues within the hobby.

The role of “starvation”. Starvation In aquaculture is used before fish movements, vaccinations, medical treatments, any handling ect. This is an effective tool that we can also use in the hobby, and many do. It is commonly used to attempt to reduce the impact of overfeeding, before transportation or to wean fish onto prepared diets. Lengths of “starvation” can last from 1 day, to several days. This is not unhealthy for most fish and is a common experience in the wild. Small periods of no feeding can be useful before or during medical treatments, before transport or before handling your fish. The role that this plays is to reduce the amount of stress caused by the action and reducing waste during times of high stress, and can be a valuable tool in aquarium management.

So, what should I feed my fish?

The first thing that I would say to many people is research, research, research. There is plenty of information out there on the nutritional requirements of fish a quick google scholar search can provide you with a whole range of information.

Once you know about the requirements of your fishes, most likely the macronutrient requirements have a look at the labels of your feeds for the proximate analysis, they even have this on the packs of frozen foods (at least in the UK!). Try and match up what your research told you with that you are feeding them. In many cases live foods don’t provide the necessary requirements by themselves and need supplementation with prepared foods. Similarly, not all prepared foods provide exactly what all fish need, and not all fish will eat prepared diets. No one feeding strategy is better than another, in many cases it depends on your fish, and their requirements. But knowing your diet and it’s proximate analysis can help you to better feed your fish.

Attached is a table taken from Royes and Chapman (2003) describing the approximate requires of the key components in fishes diet.

The Lumpfish Guy said:

So, what should I feed my fish?

The first thing that I would say to many people is research, research, research. There is plenty of information out there on the nutritional requirements of fish a quick google scholar search can provide you with a whole range of information.

Once you know about the requirements of your fishes, most likely the macronutrient requirements have a look at the labels of your feeds for the proximate analysis, they even have this on the packs of frozen foods (at least in the UK!). Try and match up what your research told you with that you are feeding them. In many cases live foods don’t provide the necessary requirements by themselves and need supplementation with prepared foods. Similarly, not all prepared foods provide exactly what all fish need, and not all fish will eat prepared diets. No one feeding strategy is better than another, in many cases it depends on your fish, and their requirements. But knowing your diet and it’s proximate analysis can help you to better feed your fish.

Attached is a table taken from Royes and Chapman (2003) describing the approximate requires of the key components in fishes diet.


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Excellent points Lumpfish Guy!

Especially the one about "portion control"
And the nutriention information vs ingredient list.
I think it's pretty much common knowledge by now among manufacturers that customers often check ingredient lists, I have read many stories about loopholes used to move an ingredient up on this list, including adding an initially high moisture content food that looks good on the list, and ordering ingredients by wet weight.

This moves the "marketing item" if you will, much higher up on the list than it would be if the ingredients were ordered by final, dry weight.

To jump categories, certain cat food manufacturers have even recently begun trumpeting thier high quality "grains, vegetables" , and even sometimes "trace fruit extract" ingredients, (my personal favorite being blueberries)
All this in a food supposedly intended for an obligate carnivore.

Checking the protein, fat, mineral, and vitamin percentages bypasses such practices altogether!

Would like to ask one thing, however.
For clarification, is the above nutritional table referring to fish in general, a specific group of fish, or a specific species?

Jeremy180 said:

Would like to ask one thing, however.
For clarification, is the above nutritional table referring to fish in general, a specific group of fish, or a specific species?

Click to expand...

This was an "in general" guideline for what you might expect. As I am sure you know the requirements are so varied between species It would be impractical to include it in the table. A quick google scholar search can bring up the macronutrient requirements for specific species. I tried to find a review article which had a list of a few common species but can't find the paper I was thinking of, If i come across it again I will post the table up.

Jeremy180 said:

To jump categories, certain cat food manufacturers have even recently begun trumpeting thier high quality "grains, vegetables" , and even sometimes "trace fruit extract" ingredients, (my personal favorite being blueberries)
All this in a food supposedly intended for an obligate carnivore.

Click to expand...

My wife is a vet and it's a real problem that people think that they can feed their cat "vegan" diets. It's animal abuse, essentially starving them of key nutrients. The BVMA are highly opposed to it.