PHOSPHATES IN RO/DI - ALGAE/CYANO ISSUES - COURSE OF ACTION

[DanielJameS]

I have been battling an issue with what I can only visually diagnose as either spirulina, some combo of that and plain ole diatoms or just typical cyano. I have certain corals that have melted (Zoa’s) and a couple LPS I’ve owned for a long time that have withered away. Long story short I’ve gone after fixing this via discontinuing biopellets, I’ve dosed Dr Tim’s Eco Balance (probiotic) as well as Waste Away and Seachem stability for the last month or so along with a couple water changes, siphoning out what I can etc. Initially I suspected a phosphate deficiency as biopellets did not ever seem to take on this tank, but i did not have a reliable PO4 kit. I just acquired my Hanna ULR Phosphate checker and my DT came back after several tests at .09ppm

Further investigation revealed my RO/DI water is testing at .11ppm (distilled drinking water is coming back @ 0. A silicates test also revealed my source water as free of those. A handheld TDS pen shows 000.

I’ve done a bunch of research on this and some of the advice appears to be “don’t worry about it” as that amount of PO4 is incidental to the overall DT Numbers via top off, but on a 125 doing larger water changes @ a starting point of .10 PO4 makes lowering those challenging. I’m running GFO (HC - BRS) but I’m evaporating a little over a gallon per day, being replaced by .10 PO4 water. In other words, my top off/water changes have obviously been holding my PO4 around .09 in the DT (before I knew my source had PO4.)

I am trying to get the PO4 down to about .04-.05 to see what impact that has on the cyano and some of the other issues I’m having. Trying to narrow down the issue to stabilize things now that I have a better overal testing picture.

I’ve read several threads of people replacing all their RO/DI cartridges to no avail, adding another DI stage etc and I hate to just throw money blindly at something I’m not sure will resolve the issue.

Any thoughts on what the best course of action would be and any ideas what could be causing these issues? Here’s some other info:

Salinity: 35ppt
Temp: 79F
NO3: 5-7ppm
PO4: .09ppm
Alk: 8.5-9DKH
Calc: (high) 540
Mag: 1350

I have a pretty good bioload, run mechanical filtration, rox carbon, and have a Coral Box D500 skimmer (which I’m starting to think is slightly undersized for bioload.) and don’t feed super heavy.

After removing biopellets, my NO3 to my surprise didn’t really rise, it’s held steady around 5-10 for the last month or so. At this point I can only suspect the PO4 as the water parameters are relatively on point, but my corals are not thriving. Some are “ok” others, not so much.

Thanks for any brainstorming in advance!

 

[rkpetersen]

Your phosphate is ok, probably not the cause of issues with corals. To clarify one thing though - is your Hanna ULR phosphorus checker reading 9 (which would be around 0.03 ppm phosphate), or are you converting the actual reading to ppm phosphate yourself? Also, are you sure of how much phosphorus is in your rodi water? Did you test it yourself (I don't know how well the checker works in fresh water), or send it in for ICP-OES testing?

If it's cyano, keep cleaning it off, keep the nutrients low but not zero, and get a refugium or algae scrubber going well. If it's diatoms, you might spring for a Spectrapure silicabuster DI cartridge for your RODI system.

 

[DanielJameS]

It’s the new Hanna ULR phosphate (HI774) so it does the ppb conversion to ppm.

.09ppm is the correct reading

 

[DanielJameS]

I think I need to go ahead and replace my RODI canisters (even though they shouldn’t be exhausted) and many times I’ve debated adding the silica buster stage (although it’s a tad pricey.) A lot of issues I’ve had could definitely be in the source water, particularly when my parameters have been mostly on point.

 

[rkpetersen]

It’s the new Hanna ULR phosphate (HI774) so it does the ppb conversion to ppm.

Edit: NM, I see there is a new meter!

 

[Toomnymods]

the new hanna checker displays in PPB (parts per billion) so if it gives you a 9 that would actually be .028PPM

 

[rkpetersen]

the new hanna checker displays in PPB (parts per billion) so if it gives you a 9 that would actually be .028PPM

No, he's right, their newest model is ULR phosphate. I hadn't seen it before this thread. Basically the same as the ULR phosphorus checker with slightly altered electronics.

 

[DanielJameS]

I determined through some deconstruction and troubleshooting that my RO membrane I was sent when I replaced it last year is not a “HR” or high rejection version (thanks Amazon.) so I’m getting more like 88% rejection before my DI which is reading about 15-16 TDS pre DI. I am getting 0 TDS at the end of the chain, but I’m wondering if there are things getting through not caught by a TDS meter, namely PO4.

Thoughts? I’m definitely going to swap that out if for nothing more than better DI resin efficiency. My source water is about 125-130 TDS and my city doesn’t use chloramines so I’m not certain if another DI stage is even necessary. I do need to acquire a pressure gauge as well because I have no idea what my incoming water pressure is.

 

[rkpetersen]

Traces of phosphorus, silicon, and other minerals can still be present in 0 TDS water. But whether it's enough to matter is another question.
15 TDS going into your DI is going to burn through resin pretty fast, and if there's any channeling or when the resin is close to being exhausted, you'll get a fair bit of breakthrough.
Low pressure can also contribute to membrane inefficiency.
With that incoming TDS I'd want to see 5 or less out of the membrane before the DI.