Using RO water in a freshwater tank

How to remineralize reverse osmosis water for aquarium use. Mixing ratios, GH and KH targets, common mistakes that crash parameters.

Reverse osmosis water comes out of the membrane with nearly zero dissolved minerals. TDS reads 0-10 ppm. GH is 0. KH is 0. pH is unstable because there's no buffer. You can't put fish in it as-is. It needs minerals added back before it goes into a tank.

The reason people use RO in the first place is control. Tap water arrives with whatever your municipality puts in it: chloramine, variable hardness, phosphates, silicates, sometimes nitrate. If your tap GH is 18 and you keep crystal red shrimp that need 4-6 dGH, no amount of conditioning fixes that. RO gives you a blank slate. You add back exactly what you want.

When RO is worth the hassle

If your tap water parameters match your fish, skip RO entirely. A fishkeeper with 8 dGH, 5 dKH tap water keeping community tetras and corydoras has no reason to bother. RO makes sense when:

You keep soft-water species (discus, crystal shrimp, wild-caught Amazonian fish) and your tap is hard
Your tap water contains high nitrate, phosphate, or silicate that fuels algae
Tap parameters swing seasonally and you want consistency
You're running a high-tech planted tank and want precise control over KH for CO2 management

The two things you must add back

RO water needs GH and KH. These do different things and most commercial remineralizers handle them differently.

GH (general hardness) is calcium and magnesium. Fish need these minerals for osmoregulation, bone development, and muscle function. Shrimp need them to build shells and molt. Plants need calcium for cell walls and magnesium for chlorophyll. Target GH depends on species: 4-6 dGH for Caridina shrimp, 6-8 dGH for most tropical community fish, 12+ dGH for African cichlids.

KH (carbonate hardness) is the buffer that prevents pH from crashing. Without KH, pH drifts wherever ambient CO2 pushes it. A tank with 0 KH can swing from 7.0 to 5.5 overnight as fish respiration adds CO2 to the water. Target KH is 2-4 dKH for planted tanks, 4-8 dKH for community fish, higher for cichlids.

Some products add both. Salty Shrimp GH/KH+ raises GH and KH together. Seachem Equilibrium raises GH only (calcium, magnesium, potassium, iron) and does nothing for KH. If you use Equilibrium alone, you still have 0 KH and your pH will be unstable. You'd need a separate product like Seachem Alkaline Buffer to add KH.

Mixing ratios

The simplest approach for community tanks: mix RO with your tap water. If your tap is 12 dGH and you want 6 dGH, mix 50/50. The math scales linearly. 75% RO / 25% tap gets you roughly 3 dGH from 12 dGH tap water.

This works well when your tap water is otherwise clean (no excess nitrate or phosphate) and you just need to bring the hardness down. It also preserves the natural mineral balance of your tap, which is harder to replicate with pure salts.

For pure RO remineralization, mix minerals into the water change water before adding it to the tank. Not after. Adding dry minerals directly to a tank with fish creates localized concentration spikes.

A common recipe for community tropical fish uses lab-grade mineral compounds:

Calcium sulfate (CaSO4): raises GH, adds calcium
Magnesium sulfate (MgSO4): raises GH, adds magnesium
Potassium bicarbonate (KHCO3): raises KH, adds potassium

The exact amounts depend on your target hardness and on the precise compound used, since hydrated forms add less hardness per gram than anhydrous forms. Work the amounts out for your own GH and KH targets rather than copying a generic figure. The remineralizer calculator gives the amounts for specific targets so you don't have to estimate.

Common mistakes

Not testing after mixing. The first few batches, test GH, KH, and pH of your mixed water before it goes near the tank. Mineral solubility varies with temperature and water agitation. What dissolves fully in warm, stirred water may precipitate in a cold bucket.

Forgetting about top-offs. When water evaporates, minerals stay behind. If you top off evaporation with more remineralized water, you're adding minerals on top of the ones that didn't leave. Top off with pure RO. Only use remineralized water for actual water changes where you're removing old water.

Chasing exact numbers. GH of 5 and GH of 7 are both fine for most tropical community fish. Stop tweaking when you're in the range. Consistency matters more than precision. Pick a formula, mix the same way each time, and your fish will adapt to whatever stable value you land on.

Storing pre-mixed water without circulation. A bucket of remineralized RO water sitting for a week absorbs CO2 from the air, which drops the pH. It's still fine to use, but test it before panicking about parameters. Aerating the bucket for an hour before use blows off the excess CO2 and stabilizes pH.

Mixing the wrong product and getting confused. Seachem Equilibrium is probably the most common source of confusion. People buy it thinking it handles everything, dose it according to the label, and wonder why their pH crashes. It only does GH. Read labels. If the product doesn't mention KH or alkalinity, it doesn't add any.

Ongoing cost

A 4-stage RO unit costs $60-120 and produces water at roughly $0.02 a gallon. Membrane replacement every 2-3 years adds about $25. Sediment and carbon pre-filters need replacement every 6-12 months at $10-15 each. The remineralizing minerals are cheap if you buy them as dry lab chemicals rather than branded aquarium products; a 1 kg bag of each compound lasts most hobbyists over a year.

The bigger cost is convenience. An RO unit produces water slowly (50-100 gallons per day for a home unit) and wastes 2-4 gallons for every gallon of product water. You need storage containers and a day's lead time before a water change. Most people who switch to RO set up a dedicated water station with a trash can or food-grade barrel, a pump, and a heater to get the temperature right before adding it to the tank.

For a single small tank, it might not be worth it. For a fishroom, breeding setup, or demanding species, it pays for itself in reduced headaches within the first year.