Your beautiful tetras may not always eat all of the food that you give them and this will slowly degrade, resulting in the formation of toxic compounds. Your fish will also produce waste and this will also be broken down to produce compounds which can be harmful to your tetras.
More reading
Natural diuretics for dogs
Liquid meal replacement for cats
The 3 gallon tank that you select for your fish will be their home for while and several things will take place in that habitat. The nitrate cycle or nitrogen cycle is one of the factors that will determine how many fish can be kept in your tank. In this cycle, waste materials are broken down into ammonia, then nitrites and eventually nitrates. All of these compounds can be toxic to your fish.