Do I really need RODI water for a reef tank?

Yes. Tap water contains chlorine, chloramine, copper, silicate, and dissolved solids (TDS) that wreck reef chemistry. Even 'good' tap water has 100+ ppm TDS; reef-safe water needs to be 0 ppm out of the DI stage. The few hobbyists who try without RODI almost always end up adding it after a frustrating year of algae blooms.

Can I just buy RODI water from a fish store?

Yes, and many reefkeepers do for years. Cost: ~$0.50–1.00 per gallon. Breakeven on a $150 home RODI unit comes around 150–300 gallons of use, which is 3–6 months for a typical 100-gallon reef.

What's the difference between RO and RODI?

RO (reverse osmosis) alone produces ~5–10 ppm TDS output. The DI (deionization) resin stage scrubs the remainder to 0 ppm. Reef tanks need the full RODI stack. Drinking-water systems often stop at RO because 5 ppm tastes fine.

Guide · 5 min read · 750 words

Sizing a RODI system for a reef tank

RODI sizing is about matching the filter's gallons-per-day rating to your weekly RODI demand. Get this wrong on the small side and you spend Saturdays watching a slow drip; get it wrong on the large side and you waste membranes.

What RODI actually does

RODI water filters remove dissolved solids from tap water in a multi-stage process. The output (when working correctly) is 0 ppm TDS — essentially pure water. You add salt mix to this water to create the controlled chemistry of a reef tank.

Without RODI, you're building a reef on top of whatever your municipal water adds: chlorine, chloramine, phosphate, silicate, copper, fluoride, and trace heavy metals. None of these scale cleanly with the reef chemistry math.

The stages

Sediment filter (5 µm or 1 µm). Removes rust, dirt, particulates. $5 cartridge, replace every 6 months.
Carbon block (2× recommended). Removes chlorine and chloramine that would destroy the RO membrane downstream. $10–15 cartridge, replace every 6 months.
RO membrane (75/100/150 GPD ratings). Removes 95–98% of dissolved solids. Membrane lasts 2–3 years.
DI resin. Removes the remaining 2–5% of TDS to zero. Replace when output TDS reads above 1 ppm. $20–30 refill.

GPD rating vs real output

The GPD rating is measured at 65 psi water pressure with 25 °C water temperature and 250 ppm input TDS. Most homes run 40–55 psi with colder water and lower TDS — meaning real-world output is typically 60–75% of the rated value.

A "75 GPD" system in a typical home makes about 45–55 gallons per day. A "150 GPD" makes 90–110. For higher flow, add a booster pump (raises pressure to 80+ psi).

TDS management

Run an inline TDS meter on input and output. Three numbers tell you the story:

Input TDS — your tap water. Baseline.
Post-RO TDS — should be 5% or less of input. If higher, RO membrane needs replacement.
Output TDS (post-DI) — should read 0 ppm. Once it shows 1 or higher, your DI resin is exhausted — replace it.

What to buy

Bulk Reef Supply's 4-stage and 5-stage RODI units are the most-recommended in the hobby. Spectrapure makes higher-end systems. AquaFX is solid mid-range. Whatever brand you pick, confirm: replacement cartridges are standard 10-inch or 20-inch (not a proprietary form factor), and the manifold accepts standard 1/4" push-fit tubing.

Water change calculator to size your weekly RODI demand precisely.

Equipment mentioned in this guide

BRS and Spectrapure RODI units sell direct \u2014 not on Amazon. But the inline dual-TDS meter you need to monitor membrane + DI exhaustion does. Brute can for the RODI reservoir.

HM Digital DM-1 inline TDS meter (dual)

Permanent input/output TDS monitor for RODI.

View on Amazon →

Rubbermaid Commercial Brute 14 gal container

Standard ATO reservoir. Food-grade plastic.