You should feed your fish... Okay, this is where I can get in a lot of trouble because fish really do need to feed all day long. So, they would benefit from feeding two, three, or even four times a day - but very small feedings. Most people's schedule doesn't warrant that type of feeding. So, fish are usually okay with just being fed once a day. That's what most fish stores and most fish professionals would recommend simply because people tend to overfeed as well.
You could feed your fish one time a day. Feed them as much as they can eat in five minutes. What that means is you put a little bit of food in the tank. You could shut off the filter so the filter doesn't remove the fish food. And then if the fish eat all the food you could feed a little bit more. You do that up to five minutes and then stop.
I prefer to feed the fish again two to three times a day. It's just better on the fish's system. It's not as taxing on their gut. You don't just load them up with food for five minutes. Just give them one small pinch of food in the morning, once in the afternoon, and that's the best way for the tank and the best way for the fish.
But you can't overfeed. If you feed and you see leftover food, and you do that three times a day, you're going to really foul your water quickly. So don't overfeed.
And mix up the diet. It's very important to replicate the diet that the fish would eat in the wild. There are different types of fish out there. There are herbivorous fish. There are omnivorous fish. There are fish that just eat shrimp. There are fish that just eat algae - those are the herbivorous fish.
So you want to replicate because those fish need the vitamins associated with the types of food that they eat in the wild. When you buy the fish ask the people what types of food these fish need to eat. Typically you'd just do a well balanced diet.
Very rarely do you have a situation where the fish would be hurt from eating the wrong types of foods. One situation would be with African cichlids that only eat algae. If they eat too much protein they get bloated and they can actually die from it. It causes a lot of bacteria in their gut because they can't break down the foods.
For the most part, most tropical fish, even salt water fish, would benefit just from a wide variety of food. You can't go wrong with that.