How do I remineralise RO water for an aquarium?

Why this happens

Reverse-osmosis water removes much of the source mineral content. Fish, plants, shrimp and biofilters still require suitable ions and buffering, but the correct balance differs between soft-water fish, livebearers, planted tanks, shrimp and reefs.

• GH-only products may not provide KH.
• Shrimp salts can be formulated for different species and substrates.
• Volume measurement errors become large in small batches.
• Evaporation top-off and water-change water require different preparation.

A remineralisation recipe is repeatable only when the starting water, batch volume, scale and product remain consistent. The label dose is a starting point, not proof of the final water chemistry.

What to check first

• What GH, KH and conductivity does the livestock require?
• Is the product intended for freshwater, shrimp or marine use?
• Is RO output quality stable?
• Are salts weighed or measured with an accurate tool?

Compare the result with the aquarium’s normal baseline, not only a generic target. Record the time, recent maintenance, new livestock, feeding changes, source-water details and any equipment fault. A short timeline often separates the original cause from secondary symptoms.

A practical action plan

1. 1. Measure the real batch volumeMark the container and account for residual water.
2. 2. Add minerals outside the aquariumMix with circulation until fully dissolved.
3. 3. Test the prepared waterConfirm GH, KH, pH after equilibration and conductivity.
4. 4. Introduce consistentlyTemperature-match and use the same recipe for planned water changes.

How to interpret the response

Prepared batches should cluster around the same measurements. Drift suggests RO membrane change, measurement error, product moisture or inconsistent volume.

Change one important variable at a time where welfare allows, then observe and retest. When several products or adjustments are made together, it becomes difficult to know what helped, what caused stress and whether the underlying problem remains.

What to avoid

Do not add concentrated salts directly onto livestock, use marine salt as a generic freshwater remineraliser, confuse top-off with water-change water or chase exact numbers through repeated dosing.

When it is urgent

A major overdose, sudden conductivity change, shrimp collapse or fish osmotic distress requires stopping additions and controlled dilution with correctly prepared water.

How to prevent it next time

Keep a written grams-per-litre recipe, calibrate scales and meters, store salts dry and test each new product batch.

Keep enough records to recognise the aquarium’s normal behaviour, water chemistry and equipment performance. Trends are more useful than isolated numbers, and early correction is usually safer than a large emergency change.