Poor water conditions often result from a failure of biological filtration to transform poisonous fish or food waste into harmless organic material. Ammonia from waste is a poison that can stress guppies, lowering their immune system and exposing them to ever present diseases in the tank.
There is much debate on the most effective form of biological filtration. The most common method is to place an aerated corner filter in the tank and put some form of biological filtration media in it. Another method is to use blank sponges attached to an air hose.
The amount of biological filtration to use is dependent on the number of guppies in the tank and how heavily you feed them. Beginners should keep only one guppy per gallon of water, and feed three times a day only as much food as they will eat in one minute.