The short answer
The dependable way to soften water for shrimp is to start with RO (reverse osmosis) or rainwater — both nearly mineral-free — and then bring it back up to a controlled hardness, either by cutting it with a little tap water or, better, by adding a shrimp remineraliser to hit a specific target. This gives you soft, stable water you can repeat every time, which shrimp value even more than a particular number.
Why the source water matters
Shrimp — especially soft-water species like Caridina — need low, steady hardness. Trying to chip away at hard tap water with chemical softeners gives shifting, unreliable results. Starting from near-zero mineral water and building up puts you in control: you decide the final GH and KH, and you can mix it identically for every water change.
The two approaches
- RO or rainwater cut with tap. Blend mineral-free water with a measured amount of your tap water until the hardness reads where you want it. Simple, but only as consistent as your tap supply.
- RO/rainwater plus a remineraliser. The preferred method for serious shrimp keeping. Add a dedicated shrimp mineral product to pure water to reach a precise GH (and KH for the species that need it). Repeatable and stable.
Measure as you go — a hardness test (GH/KH) or a TDS meter tells you when you’ve hit your target.
Getting it right
Decide the parameters your shrimp species wants, then mix to hit them consistently — see what pH do shrimp need? for the wider target, and should I use RO water? and is rainwater safe? for the source-water side. For the general method see how to do a water change and how to soften aquarium water. A GH/KH test kit lets you dial in and repeat your mix.