Before you start trying to treat the illness you need to work out what the problem is. I wouldn't recommend throwing melafix in and seeing if it fixes it immediately. I'm going to make some assumptions because there is little to go on.
The first thing to note is that fish of these sizes need very little food at all. I have some zebra danios and I feed them sparingly, sometimes not at all on some days. If you think about how big their stomachs could possibly be they probably only need a flake to fill them up. Be careful with overfeeding. This is classic mistake by new fish keepers. Your fish will always look hungry even when they aren't and a hungry fish is a health fish. If they aren't that excited by food, they're probably overfed.
Secondly, you need to test your water to ensure there isn't any problems with it. Buy a test kit that tests for ammonia, nitrite and nitrate and check all of the levels. The first two should read 0. If they don't, your tank is not cycled. The nitrate should be less than about 40ppm.
Finally, make sure you're doing your maintenance. Your tank needs to be heated to the correct temperature, receive regular water changes and have a filter to keep the water circulating and ensure the bacterial cycle is working.