Aquatic Life Classic Dual Deionization SystemAquatic Life Classic Deionization 10 Inch System with Color Changing Mixed Bed Resin Cartridges for Aquariums. Water purity is usually measured by the amount of Total Dissolved Solids (TDS) in the water. After water is filtered by an RO Membrane (not included), there is usually a small amount of TDS left in the water for aquarium applications it is important to remove the remaining TDS from the water. This is accomplished by filtering the water