Ideally, Tetra AquaSafe should be used in new aquariums before you add your fish but should also be used when you change the water or add more due to evaporation. The amount of Tetra water conditioner you use can depend on the variety. Still, instructions for the correct ratio of water to conditioner should be on the back of the bottle. Add the solution to replacement water at the same temperature as your aquarium water and wait for your Tetra aquarium filter to fully cycle all the water in the habitat.