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How to choose
Reef tanks start with pure water. An RO/DI system strips tap water down to 0 TDS so you control everything that goes into the tank. The main choice is stage count, which depends on your water. Four stages suit low-chlorine city water or clean well water. Five or more add a second carbon block for chloramine. Output (GPD) depends heavily on water pressure and temperature.
- Find out whether your utility uses chloramine. If it does, buy at least 5 stages.
- Watch the TDS meter after the RO membrane and after the DI. Replace DI resin when the reading rises above 0.
- Low household pressure (under about 50 PSI) cuts output and raises waste. A booster pump fixes that.
For the science behind it, read our Salinity and RO/DI guide.
How we review: we paraphrase what owners report in retailer reviews and forum threads, plus picks and test results from BRStv, Reef2Reef discussions and published reef chemists. We only show a rating when we verified it at the source, with the date we checked. We don't invent scores or quotes. Prices and stock change, so check the retailer.
Well-regarded picks
BRS 4 Stage Value Plus 75 GPD RO/DI System
4-stage RO/DI
- What reviews say: strengths
- Owners highlight easy setup, the included pressure gauge and dual TDS meter, and steady 0 TDS output.
- Weaknesses
- A few buyers had dripping push-fit connectors out of the box. Some wish the shutoff clamp and other fittings came included.
- Who it's for
- Most first reef tanks on low-chlorine city water or clean well water.
- Expert consensus
- BRS's 2026 RO/DI guide rates 4-stage units "good" for low-chlorine city water or clean well water, and the most affordable and portable option. Its BRStv buyer's guide explains how to match stages to your source water.
Sources
Where to buy:
BRS 5 Stage Premium Plus 75 GPD RO/DI System
5-stage RO/DI
- What reviews say: strengths
- Reviewers like the before-and-after TDS meters, a simple install, and 0 TDS water. Several beginners said the BRS videos made setup easy.
- Weaknesses
- One reviewer found changing the membrane frustrating with little written instruction, and one had a single leaky fitting.
- Who it's for
- City water treated with chloramine, which is common in the US.
- Expert consensus
- BRS calls its 5-stage line the best-selling option and the least expensive one that handles chloramine, thanks to its second carbon block.
Where to buy:
BRS 4 Stage Value Plus 150 GPD Water Saver RO/DI System
4-stage RO/DI, water-saver membrane
- What reviews say: strengths
- Owners report faster production and less waste water than older units, and like the flush valve and auto-shutoff.
- Weaknesses
- Real output depends on pressure. One reviewer at about 60 PSI got about 100 GPD, and one had TDS creep after a couple of months.
- Who it's for
- Bigger systems or frequent water changes where speed and waste matter.
- Expert consensus
- BRS's guide treats water-saver membranes and output speed as the main reasons to step up, alongside source-water quality.
Where to buy:
Spot an error or a product we should add? Specs and availability change often, so always check the manufacturer's current listing.