Can I use tap water in a reef tank?

It is strongly discouraged. Tap water can carry phosphate, nitrate, silicate, copper and other metals, chlorine and chloramine - the exact things that fuel algae, feed pests, and stress or harm corals and inverts. Most reef problems traced to source water disappear once keepers switch to RODI.

RODI is reverse-osmosis, de-ionised water - tap water pushed through a sediment filter, carbon block, an RO membrane and a DI resin stage, which strips out dissolved solids down to near zero. You then mix salt into it for saltwater, or use it plain to top up evaporation.

What TDS should reef water be?

Aim for 0 TDS out of your RODI unit - a cheap inline TDS meter should read 0 (or very close). A rising TDS reading means a filter stage or the DI resin is exhausted and needs replacing. TDS is your simple, honest check that the water is actually pure.

Should I buy RODI water or make my own?

Both work. Buying RODI from a local fish store is fine for small or nano tanks and low volumes. An RODI unit at home pays off as your water use grows, gives you water on demand, and lets you verify purity with a TDS meter - most keepers with more than a nano end up making their own.

Do I use RODI for top-off and water changes?

Yes to both - but differently. Top up evaporation with plain (saltless) RODI, because only water evaporates and the salt stays behind. For water changes, mix salt into RODI to your target salinity first, and never add unmixed or unaged saltwater to the tank.

RODI Water for Reef Tanks (Why It Matters)

🐠 The short version: reef tanks use RODI (reverse-osmosis de-ionised) water, not tap, because tap water carries phosphate, nitrate, silicate, metals, chlorine and chloramine — exactly what fuels algae and pests and stresses corals and inverts. RODI strips those to near zero (aim for 0 TDS). Use plain RODI for evaporation top-off and salt-mixed RODI for water changes. Buy it or make it — just verify it’s pure.

🔧 Mixing a batch? The salt-mix calculator gets you to target salinity, and the salinity converter checks your reading.

Why not just use tap water?

Tap water is treated to be safe to drink, not to run a reef. Depending on where you live it can carry:

Phosphate and nitrate — fertiliser for nuisance algae and cyano.
Silicate — feeds diatoms (a big driver of the new-tank “ugly stage”).
Copper and other metals — toxic to inverts and corals (copper especially).
Chlorine and chloramine — disinfectants that harm livestock and your biofilter.

A huge share of “mystery” algae blooms and struggling corals trace straight back to source water. Starting with pure water removes an entire category of problems before it begins.

What RODI actually is

RODI water is tap water pushed through a series of stages:

Sediment filter — removes grit and particles.
Carbon block(s) — strips chlorine/chloramine and organics (and protects the membrane).
RO membrane — the workhorse: rejects the large majority of dissolved solids.
DI resin — polishes the RO output down to near-zero dissolved solids.

The result is water with essentially nothing in it — a clean slate you then mix salt into (for saltwater) or use plain (for top-off).

TDS: your simple purity check

A cheap inline TDS meter (total dissolved solids) is the honest gauge. Fresh RODI should read 0 TDS (or very close). When the number starts rising, a stage is exhausted — usually the DI resin first — and it’s time to replace it. Checking TDS turns “I think my water’s clean” into a number you can trust.

Buy it or make it?

Buy RODI from a local fish store — perfectly fine for nano and low-volume tanks.
Make your own with a home RODI unit — pays off as your water use grows, gives water on demand, and lets you verify purity yourself. Most keepers past the nano stage end up making their own.

Either way, use RODI for both jobs: plain (saltless) RODI to top up evaporation (only water evaporates, so no salt goes in), and salt-mixed RODI for water changes — never add unmixed or unaged saltwater to livestock.

The honest takeaway

Getting the water right starts before the salt goes in. Pure RODI water removes the phosphate, silicate, metals and chlorine that quietly cause algae, pests and coral stress — so it’s one of the highest-leverage habits in the hobby. Keep a TDS meter on it, replace stages when the number climbs, and build every water change and top-off on a clean foundation.

Sources & further reading

Coralkin — Reef tank water chemistry explained — where source water fits in the bigger picture.
Coralkin — Saltwater water changes & maintenance — top-off vs water change, done right.
Coralkin — Salt-mix calculator and salinity converter — mixing RODI to target salinity.
RODI / source-water references (Bulk Reef Supply water-treatment guides) per our chemistry guides.