Reverse osmosis and rainwater are blank canvases. Both strip out almost everything, leaving water with next to no dissolved minerals — brilliant for consistency, and lethal to shrimp if you forget the second half of the job, which is putting minerals back. This guide is the how-to for that second half: what to remineralise with, how much, and how to dose to a number instead of a hopeful guess. Get it right and you'll build identical water every single time, which is more than most keepers on tap can say.
Why start from RO or rainwater at all
Most keepers reach for pure water for one of two reasons. The first is soft tap water. If you're in Scotland, Wales, Cornwall or the North West, your supply likely runs at GH 2–6, which isn't enough mineral for reliable moulting — the region-by-region picture is in our UK tap water guide. You can remineralise soft tap directly, but many soft-water keepers prefer to start from a known blank and build up, because it sidesteps whatever else the tap happens to carry that week.
The second reason is control. Water from a reverse osmosis unit reads close to zero dissolved solids whatever the season, so every batch you mix comes out the same. For a breeding project chasing steady conditions, that repeatability is the whole appeal — you decide exactly what goes into the water rather than inheriting it from the mains.
Neocaridina don't need RO the way Caridina do, mind. This is a tool for soft-water regions and for keepers who want batch-to-batch consistency, not a default everyone should adopt. Hard-water keepers rarely need to touch it.
Pure RO or rainwater will kill shrimp on its own
This is the point that catches people out, so we'll be blunt about it. Water with a TDS near zero is not "clean" water a shrimp can live in — it's water with nothing in it, and a shrimp dropped into it fails fast on two fronts.
First, there's no calcium or magnesium to build a shell from. GH measures exactly those minerals, and they're the raw material a shrimp draws on to harden a new exoskeleton every time it moults. At GH 0 there's nothing to pull on, so moults stall — you get the failed moults and the white ring of death that end more soft-water colonies than any disease. The mineral mechanics behind that are in failed moults and the GH connection.
Second, there's no buffer. KH is the carbonate hardness that holds pH steady, and pure water has none, so the pH can crash overnight as the tank's own biology acidifies it. A colony can look fine at bedtime and be gone by morning. Never, ever add unremineralised RO or rainwater straight to a shrimp tank. It always gets minerals put back first.
What to remineralise with
You want a shrimp-specific remineraliser: a powder you dissolve into the water to raise its hardness in a controlled way. There are two broad types, and the distinction matters.
| Type | What it raises | Best for |
|---|---|---|
| GH+ | General hardness only (calcium, magnesium) | Tanks that already have some KH, or where you buffer separately |
| GH/KH+ | Both general and carbonate hardness together | Building water from pure RO or very soft rain — the safe default |
For Neocaridina built from RO or rainwater, we favour a GH/KH+ remineraliser, because it puts back both the shell minerals and the buffer in one step. A GH-only product leaves KH sitting near zero unless you add carbonate separately, and near-zero KH is the pH-crash risk from the section above. A combined GH/KH+ powder closes both gaps at once.
Whatever you use, it should be a product sold for shrimp or aquarium use — the dose is designed around it. This isn't the place for garden minerals, home mixes or anything improvised; the whole point of buying pure water was precision, and you keep that precision by adding a known powder to it.
Dose to a number, not a guess
Here's the method we use, and it's the same whether you're remineralising RO, rainwater or soft tap.
Start with your pure water. If it's tap or rainwater rather than RO, dechlorinate it first — RO from a good unit needs no dechlorinator, but tap always does. Add the remineraliser a little at a time, stir to dissolve it fully, and measure the TDS with a pen. Keep adding and stirring until the TDS reads where you want it, then stop. You're aiming for TDS 150–250, with GH landing in the 6–12 range as you get there; for breeding tanks, nudge towards TDS 180–250 and GH 8–12.
The reason we dose to TDS rather than by the scoop is that it takes the guesswork out. A TDS pen costs little, reads instantly, and turns "about right" into an actual number you can hit again next week — which is the entire reason for using RO in the first place. There's more on reading and trusting that number in our TDS guide for shrimp keepers, and the full target table sits in the Neocaridina water parameters guide.
Mix a batch, let it dissolve and settle, and check the TDS has held before it goes anywhere near the tank. Then treat it exactly like any new water: matched to tank temperature and added slowly, never as a cold slug all at once.
Rainwater: the honest version
Rainwater is free, naturally soft, and tempting for anyone in a soft-water area who'd rather not run an RO unit. Some keepers grow beautiful colonies on it. We've never quite trusted it enough to bother, and it's worth knowing why before you set out a butt to collect it.
Rain is only as clean as everything it touches on the way to your barrel. Off a roof, it picks up whatever's on the tiles — bird droppings, moss treatments, road dust, the residue of any moss or algae killer used up there — plus whatever the local air carries down with it. Collection surfaces matter enormously, and the first flush of a downpour, which rinses the dirtiest material off the roof, is the batch you least want. Pesticide and pollutant traces don't announce themselves; a TDS pen tells you the mineral load but not what's dissolved in it.
If you do use rainwater, collect from as clean a surface as you can, divert the first flush away, filter it, store it dark, and remineralise it to a TDS number the same as you would RO. Even then, we'd treat a first tank as too precious to gamble on it. Given the choice between free-but-variable and cheap-and-consistent, most serious soft-water breeders we know quietly settle on RO.
Fitting it into the routine
Remineralised water isn't only for filling a new tank. Every water change and every top-up in a soft-water setup uses it too, so it becomes a standing part of the week rather than a one-off. Mix your change water, remineralise to your usual TDS, temperature-match it, and add it gently, exactly as covered in our guide to safe water changes for shrimp. And when you first set a tank up, remineralise from the very start so it cycles at the hardness it will run at — no mineral jump on the day the shrimp arrive.
Do that, and RO stops being an intimidating extra step and becomes the most boringly reliable water in the hobby. The reliability is the whole point, and it folds straight into the wider routine in our cherry shrimp care guide.
FAQ
Can cherry shrimp live in RO water?
Not in pure RO — that would kill them. RO water has almost no dissolved minerals, so there's no calcium or magnesium to build a shell from and no buffer to hold pH steady. Remineralised RO, on the other hand, is excellent: add a shrimp GH/KH+ powder to bring it up to GH 6–12 and TDS 150–250 and you've made water many breeders prefer for its consistency.
How do I remineralise RO water for shrimp?
Add a shrimp GH+ or GH/KH+ remineraliser to the water a little at a time, stirring to dissolve it, and measure with a TDS pen as you go. Stop when the TDS reads 150–250, which lands GH in the 6–12 range. Dose to that number rather than by guesswork, then temperature-match the water and add it to the tank slowly. For building from pure RO, a GH/KH+ product is the safer choice.
What GH and TDS should remineralised shrimp water be?
Aim for GH 6–12 and TDS 150–250 for a general colony, tightening to GH 8–12 and TDS 180–250 if you're breeding. GH supplies the minerals for moulting and TDS is the fast, cheap number you actually dose to. Keep whatever you land on consistent batch to batch — the point of using RO is that steadiness, and it's undone if every mix comes out different.
Is rainwater safe for a shrimp tank?
It can be, but it's a gamble unless you're careful. Rain picks up contaminants off your roof and out of the air — bird droppings, moss killers, pesticide and pollutant traces — that a TDS pen won't reveal. If you use it, collect from a clean surface, divert the first flush, filter it, and remineralise to a TDS number as you would RO. Many soft-water keepers, ourselves included, find RO more reliable and less worrying.
Do I need to dechlorinate RO water?
RO water from a working unit has had chlorine and chloramine stripped out along with everything else, so it doesn't need dechlorinating. Rainwater doesn't either. The exception is if you're remineralising soft tap water rather than RO — that still carries chlorine or chloramine and must be dechlorinated first, every time, before you add the remineraliser.