avoid giving too much food to fishes. they tend to eat more and finally die out of indigestion. follow feeding twice a day.

I believe you should check if there is proper oxygen available in you tank vis-a-vis the size of your tank. also good filtration system would help you to keep the water clean. incase you are a beginner pls have more of hard fishes - chichlids and seek help of the an experienced person / the outlet you have bought your fish tank for cleaning and changing of water (recommended once in 7-10 days..only take out 1/4th and add fresh water along with chlorine,anti itch and general aid medicines) . also you would need to see what type of food is your fishes most interested in having.. you can try combining blood worms and Terta bits (make sure get the orginal one - made in germany). Pls try and give then adequate fooding in two cycles and check if all of them are eating properly. also combination of fishes are important. currently go for Hard fishes only like - black sharks, goramies etc.. would suggest not to buy expensive fishes at this moment..do not put in too many fishes together..

If you're a beginner then you can try for gouramis/chichlids which are hardy fish. Also check for filtration system.