Measurement
Reverse osmosis unit
The tool that makes soft-water species practical in a hard-water region. RO water on its own is unsuitable for an aquarium: it has no buffer and no minerals, so it must be blended with tap water or remineralised to a target GH and KH.
Short answer
A membrane filtration system that produces near-pure water, used to dilute hard tap water down to a target hardness — after which it must be remineralised.
What it does
- Removes dissolved minerals, nitrate and most contaminants from tap water
Strengths and where it fits
Strengths
- Makes soft-water and blackwater species achievable anywhere
- Removes source-water nitrate
- Gives complete control over water composition
Good fit for
- Soft-water species in hard-water regions
- Caridina shrimp keeping
- Areas with high tap-water nitrate
Limitations and where it is the wrong choice
Limitations
- Produces several litres of waste water for each litre made
- Membranes and prefilters need replacing
- Pure RO water must never be used undiluted
- Slow — plan ahead for water changes
Poor fit for
- Keepers whose tap water already suits the species they want