Even when you have good biological filtration in your tanks, your guppies can still get sick. The culprit is usually high levels of nitrates or organic material in the tank. Organics include fish waste transformed into a non-poisonous form by biological filtration. Also present are nitrates, which is the end product of the biological cycle. Nitrates can be stressful to guppies in high concentrations. Disease organisms can feed off the organics, increasing their number and overwhelming the guppy’s natural defense systems. The solution is to change a percentage of the water every week or a couple of times a week.
If you do not feed your guppies heavily and your tanks have only one guppy per gallon, you can do 30% water changes once a week. In tanks that are crowded and where you are feeding heavily, two 30% water changes per week are recommended.
Feeding a high quality food that is easily digestible is an important way of ensuring that the food does not pass through the guppy and become harmful waste.