Are fish foods all of a similar quality, or are some better than others — and would it be a life-or-death situation if, for example, you fed tropical fish on a food designed for coldwater fish, or vice versa?
A) Neale replies: This is a tough question to answer. Partly, it will depend on what sort of fish you are keeping. Let’s say you’re keeping Guppies: while the flake will provide the core of their diet, they’ll also be consuming algae, decaying plant material, and even organic detritus they find on the substrate. Chances are that whatever shortcomings there might be with the flake food offered, they’ll get a decent supply of vitamins, minerals, and dietary fibre from other things they’ll be eating in the tank. On the other hand, you might be keeping more carnivorous fish that consume nothing beyond the flake food provided. Hatchetfish, for example, will fall firmly in that camp simply because they have jaws adapted exclusively for taking food from the surface, so the flake food you offer them has to cover every single one of their dietary needs. As such, you’d be justified to play it safe and choose the best possible food you can.
The other factor, of course, depends on whether flake is the only thing you’re offering them. If it is part of a varied selection that also includes such things as frozen bloodworms and live brine shrimp, then again, the flake doesn’t have to be a complete diet in itself. Even varying between, say, generic flake food, micropellets, and algae-based flake foods, will go a long way towards ensuring your fish have a good diet. It’s so easy to feed tropical fish with food from the fridge — whether fresh greens, cooked peas, or tiny amounts of seafood and white fish fillet — that even on a tight budget you shouldn’t have any problems keeping your community tank healthy.
Most flake foods are made from fish meal together with various additives that improve shelf life and palatability. There may be some variation between brands, of course, and there certainly are flake foods designed expressly for particular sorts of fish. If you’re keeping largely herbivorous fish, then the algae-based flake foods probably are a better bet than standard flake, and similarly, the flake foods designed for carnivores are usually taken more willingly than other types of flake. Colour-enhancing flake foods can be very useful with certain types of fish, particularly those with red colouration.
So far as coldwater flake food goes, I’ve used it with tropical fish at times when I had nothing else at hand, and never seen any sign of trouble.
That said, the formulation of goldfish food tends to veer more towards nutrients that are easily digested at lower body temperatures, and one difference you can see is that goldfish food contains more oil and less fat that tropical fish food. At lower temperatures, fats are harder to digest, whereas oils, being liquid already, can be absorbed much more readily. Some goldfish foods are richer in plant material than typical tropical fish food designed for more carnivorous species like tetras and danios.
Overall, yes, there are differences, and giving tropical fish foods to goldfish kept at low temperatures could be a problem, while coldwater flake food might not match the requirements of small carnivorous fish particularly well, and if too ‘planty’ in flavour, might not be accepted at all. On the other hand, herbivorous to omnivorous species like Mollies and barbs probably wouldn’t care either way if such foods were used in the short term until you had a chance to pick up a tub of their usual, preferred tropical flake.