Need RODI system / Phosphate Help

Jake Martensen · Mar 27, 2025

I have a 400g reef. Started loosing corals. Checked parameters and everything was consistent but my phosphate was .9 on a hanna checker. So I did the normal thing and did a few large water changes. No impact. Corals continue to die.
After cleaning, reduced feeding etc I finally checked my water change water. .9 on the hanna checker. grrr
I replaced all media in my 6 stage 100 gpd RODI.
1 sediment, 1 x 5 micron carbon, 1 x 1 micron carbon, 1 Cation, 1 Antion, 1 mixed bed resin all from BRS. Phosphates in the RODI water dropped to .35 and TDS is 0.
So i replaced the 2 x 100 gpd membrane with 2 x 150 gpd membranes this evening.
Let it run for an hour and tested the fresh RODI water on a hanna checker.
.63 phosphates.

I am stumped and looking for advise. I am happy to drop back to the 100gpd membrane if that will help. I just cannot be using high phosphate water for water changes, defeating the purpose.
Thanks for any advice you have!!

brclark82 · Mar 27, 2025

Is it possible something is leaching phosphates into your rodi water after the tds reading? I don’t think it’s possible to have 0.35 phosphates in 0 TDS water. It is possible to have a small amount but not 0.35 or higher.

I could be wrong about that but maybe make sure something post rodi/tds measurement isn’t adding the phosphate

Shirak · Mar 27, 2025

Will the hanna read phosphate correctly in RODI water? I would mix up some fresh salt water and try testing that to see what you get.

I have no clue but even if it is correct the amount of phosphate you are adding to the tank via water change is not as bad as it sounds. A 10% (40gal) water change at .6ppm phosphate diluted into 360gal is factor of 9 reduction. So something around .07ppm added to the tank.

@Randy Holmes-Farley is much better at explaining it than I am! I hope I am understanding it correctly.

End of the day I just do water changes and monitor the tank and do what I need to do on the tank to keep things where I want them.

Jake Martensen · Mar 28, 2025

brclark82 said:
Is it possible something is leaching phosphates into your rodi water after the tds reading? I don’t think it’s possible to have 0.35 phosphates in 0 TDS water. It is possible to have a small amount but not 0.35 or higher.

I could be wrong about that but maybe make sure something post rodi/tds measurement isn’t adding the phosphate

The water does from the RODI unit through about 6 ft of 1/4 water line into the food grade water mixing barrel. I got the water dripping directly from the shut off float.
When mixing the hanna reagent it does have a slight blue tint which is consistent with having high phosphates in my experience.
I will take apart the rodi unit and check all the filters, repack the resin media and check the membranes.
Any other thoughts are welcome!

Shirak · Mar 28, 2025

Can you collect and test a sample right after the DI stages before going through the tubing to the storage container? Let it run a few minutes first. It might also be worth testing your source water to see what level you are starting with.

Kenmx10 · Apr 21, 2025

Im thinking its a bad batch of reagents. Im having the same issues. My Phos usually read around .03-.05. Got a new batch of reagents off Amazon and the water after adding the reagent turns dark blue and I get a flashing reading of .9 on my Hanna Checker. I checked it with a Salifert kit and it was reading below .03.

Kenmx10 · Apr 21, 2025

Took another sample using a reagent from a different pack and got a reading of .03 which is normal. Did 4 test with the new pack, all 4 read a flashing .9 and the water turned blue.

Im wondering if maybe Hanna got the reagent mixed up. I only use the ULR Phos checker. Does anyone use a different checker where the water turns blue after adding the reagent?