How can I reduce phosphates in RODI H20, Nitrates in tank, and Green Algae Spurts?


Premium Member
I use RODI H2O for my 125 gallon combination fish and coral saltwater tank. I have 125 lb live rock and live sand inside. I test the TDS of the H2O that I just make (prior to mixing salt) and get a TDS reading of zero; however the measured Phosphate reading is at 0.5. Additionally my Nitrate reading continues to read about 10-15; never dropping lower. I have been using Phosphate absorbing media bag that I place in the sump where there is high water flow. I also have flourishing green algae that seems to grow wild within a week to two. This requires aggressive and time consuming brushing of live rock and cleaning sand bed and performing weekly 15 â€"œ 25% water changes.

I also can't understand why I have Phosphate with a Zero TDS reading?

Does anyone have any suggestions on fixing any of my key issues to get my phosphates down, nitrate lower, and reduce green algae spurts?

Any information would be greatly appreciated.


Premium Member
Well, either your tds meter is off or your PO4 test kit is bad. You should not test any PO4 in a properly functioning ro/di unit.

Since you have microalgae, you have elevated phosphates. The bag in the sump may not be enough.

I run my GFO in a canister filter hanging off the back of the tank and it keeps my PO4 undetectable. You may just have to get a little more aggressive.

As far as the nitrates, they are not too bad but give this a read.

Nitrates in the reef aquarium


New member
Your nitrates are within acceptable limits for a reef tank. If you have the room in your sump maybe a refugium can be added. This will help with both nitrates and phosphates.


Premium Member
I bought a two little fishies phosphate reactor with 150 grams of phos-ban. I change this out about every 4 months, and have no issues with phosphates whatsoever.