For the most part most fish tend to “graze” and eat over the course of a day or night rather than have a set feeding time. It could be considered they are opportunistic eaters, if food drifts by or they happen upon it, well, that is as good as time as any to eat.
For obvious reasons this is hard to mimic in an aquarium, generally you should try to feed less more often. Fish can be gluttonous and will often eat too much which is not really a healthy way of eating.
Whatever is on the menu also tends to be what they will eat so if you are not feeding appropriately, you can inadvertently be killing your fish. If an herbivore for instance has too much protein from animal-based food, it will have trouble digesting and become ill and long term most likely die.
Fish need carbohydrates, fats, proteins and minerals and vitamins. How much of each food type they need depends entirely on the species, and whether they are a herbivores, carnivores or omnivores or Insectivores.
Know what diet your fish would naturally follow and mimic it as best you can. That is to say, for example, if your fish is a herbivore use plant based food products for the vast majority of it’s diet. Same goes for each type of fish even Omnivores who require a balanced diet.
Do not mix different eaters in the same tank for extended periods of time as it will make it extremely difficult to provide a consistent and “friendly” diet.
Do not over feed, this will often lead to poor water quality, unhealthy fish from overeating and unwanted bacteria that can lead to fungal diseases. Try to feed small amounts of food more often rather than a large amount once a day.
Due to that fact that Herbivores typically consume plant matter that is difficult to digest as it contains a high % of fibre they have much longer digestive tracts than carnivores for instance.
Herbivores also don’t really have a stomach as such, and the food is instead broken down in the intestines and their natural diet would consist of algae, vegetables, fruits and plants.
To assist in breaking down the food they would typically grind their food with flat teeth, much like a cow chews cud.
There are very few true or totally herbivore fish and for the most part fish that are classified as herbivores can consume small amounts of meat. In fact, it is beneficial in small quantities as it provides needed minerals and vitamins.
A well planted tanks with algae is a great way of providing a base diet however it is likely that it will need to be supplemented with other foods such as raw or blanched vegetables like Brussel Sprouts, Beans, Zucchini, Cucumber, Spinach, and the like. Just about any fruit or vegetable… Make sure they are cleaned, and a good idea is to blanch to eliminate pesticides and other chemicals that can be applied during commercial growing. If possible, by organic produce to minimise this risk.
To help balance the diet and provide some additional vitamins and minerals and proteins occasionally feeding a well-balanced Omnivore flake, gel or pellet food.
Omnivores can consume both meat and plant matter as they possess the capability to process both sources of nutrients.
In many ways Omnivores are much easier to keep healthy and happy from a feeding perspective as they can gain their requirements from multiple food groups. It is important though to make sure that protein does not make up more than 35% to 40% of their diet.
Most fish will consume insects that fall into the water from the vegetation above and or the larvae from water breeding insects like mosquitoes. Insectivorous fish primarily feed on aquatic insects, terrestrial insects, insect larvae and pupae.
Many fish food products on the market lack insects as they are hard to breed, harvest and slow dry. Try to find products that specifically note that they include some form of insect meal.
In the wild insects, larvae, pupae, and terrestrial insects provide a complex range of minerals, vitamins, proteins, and fibre.
Fish in this category require a large amount of meat. Naturally this would come from live and dead sources. Most carnivorous animals including fish typically have larger mouths and pointed teeth which helps them rip apart their prey.
These fish require high amounts of protein up to about 75% and then a balance of fats and carbohydrates to be healthy. Mostly they will get the fats from the main food source and carbohydrates from a small amount of plant matter. Typically they are not designed to process large amounts of carbohydrates so this should be avoided.
Most aquarium fish adapt well to eating frozen foods like brine shrimp, blood worms, micro worms, tubifex worms and daphnia. If you are set up to hatch brine shrimp this is a particularly good source of live food. Feeder fish available from aquarium stores can also be good however they are not always the best choice as they are bred on mass so have little nutritional value and can harbour unwanted diseases.