If you find your fish are on the bottom of the tank and they're not eating, probably there's something going on. Typically, there's water chemistry, but your water could be fine and the fish could be sick.
So the first thing you want to do is try to look for symptoms on the fish. That will be the easiest way to diagnose what the problem is. Look for body slime, white spots, clamped fins, maybe the fish are getting chased. If it's only one or two fish that are eating and the other fish aren't, it could be due to aggression and stress. It affects fish just like it affects people. But if all the fish aren't eating, and they're all staying low to the bottom, typically they'd be breathing heavy, you really want to look at your water chemistry. You want to test your pH. Your temperature should be within the appropriate range for the fish you're keeping, which is usually 76 to 79 degrees for most tropical fish. So test the water. Make sure the ammonia, the nitrates, all those levels are where they're supposed to be, which is very, very low. And the pH is also supposed to be within an appropriate range.
If you have a salt water aquarium, you want to also make sure your salinity is where it's supposed to be. And remember that salt water fish are a little bit more sensitive than fresh water, so your water could be perfect, but it could be just the fish is distressed. It could be a new fish going in a new tank, and a lot of times they don't eat. They stay at the bottom. It takes a lot out of a fish to go from a wild environment. Especially the larger fish; they've been used to eating and doing their thing in the wild for so many years, and when they get put into an aquarium, it's a big jump for the fish. So a lot of times, too, just give them some time. But certainly if the fish are not eating, you always want to test the water, make sure that you're doing everything you can for your aquarium.