From what I have read and told to me by fish and animal companies, one needs 5 gallons of water for every inch of fish. I don't know if that is squared or not but I would start there and make sure they get good air flow that moves the water to help in keeping it healthy for the fish. Just hillbilly tricks I have used over the past 20+ years. Plus fish just die from stress due to being moved from the store to a new tank. A lot of things could be going wrong but not on your end. I had a fish kill in my tank that took out all my guppies and mollies but one. I still haven't figured out what went wrong. Hope this helps good luck and take care.

Your question.... if u can call it that is kinda all over the show. What size tank do you have? what fish are you keeping and how many? and what is the problem? is it the new fish getting stressed when bringing them home from the store? if so there are a few tips on how to introduce new fish into your tank (Stress free!) no hillbilly tricks.... lol

There are ways to adjust fish to a new tank environment, but it could also be from preexisting fish conditions and diseases from whatever store you got the fish from. Always buy really healthy, lively fish, and watch out for common diseases like Ick (little white spots or grains on the fish's body) and swollen eyes or excessively red gills. Don't buy fish from a tank with a dead fish in it (probably common sense.) It could have that fish's disease as well. As for your fish attacking and eating guppies... Well, that's not unusual. Guppies are tiny little fish, and aggressive, larger fish will probably see them as food. It's too late to train your big fish to get used to them now. It'd be helpful if you were more specific about the problems in your tank. Maybe I could give you a better answer, but here's here's some suggestions.