Tap Water
Contains chlorine or chloramine — always dechlorinate.
Most municipal tap suits community fish after treatment.
Check utility website for mineral content and disinfectant type.
Well Water
No chlorine but variable hardness and possible iron or heavy metals.
Test extensively before use.
May need aeration to drive off CO2 and raise pH.
RO / DI Water
Pure H2O — need to remineralize.
Essential for soft-water species (discus, wild bettas, many South American cichlids).
Home RO units cost $100 to $300.
Rainwater
Naturally soft and acidic.
Risk of airborne contamination.
Use only in rural locations and filter before use.
Mixing Sources
Common to blend tap with RO for target hardness.
E.g. 50/50 tap and RO halves hardness.
Test every batch to confirm.