Demineralised water and remineralisation
Osmosis water is pure but empty of minerals. In freshwater aquariums it must be remineralised with specific salts to achieve the desired GH and KH.
Detailed explanation
Reverse osmosis (RO) water is free of minerals. For marine aquariums, synthetic salt is added. For freshwater aquariums it is remineralised with specific salts.
The most used products in freshwater aquariums are: Salty Shrimp GH/KH+ (for fish), Salty Shrimp GH+ (for Caridina shrimps, raises only the GH without raising the KH), Seachem Equilibrium (raises only the GH).
Remineralization is done by measuring TDS with a TDS metre. For Neocaridina the target is TDS 150-300. For Caridina TDS 100-150.
Never use pure osmosis water for fish: the total absence of minerals causes osmotic shock. Even minimal remineralization (GH 3-4) is sufficient.