Aquaforest NO3 test turning red

[Shirak]

The tank is about two years old, it started with dry and a few seed live rocks, no sand.
About the phosphates, I even dosed them for a few months when nitrates measured in the 30-50 with a salifert test, I dosed about 0.02 a day and regularly measured with the tropic marin test kit. There was no nitrate reduction and in the end I also tried adding a bit of carbon dosing (diy nopox) and got a huge cyanobacteria bloom.
My guess then was the the cyanobacteria was using the phosphate more efficiently than the "good" bacterial population and when the tank got stable again did a chemiclean cycle followed by bacteria dosing (mb7), but the cyano came back and the nitrate remained in the 30-50 range.
Honestly I don't know what else I could try.

How many gallons is the system? Are you able to do a couple massive water changes? Ie replace just about all of the water...
How about chaeto refugium in the sump with lights and phosphate dosing during the night when tank lights are off?

 

[Aleph]

How many gallons is the system? Are you able to do a couple massive water changes? Ie replace just about all of the water...
How about chaeto refugium in the sump with lights and phosphate dosing during the night when tank lights are off?

That's a good suggestion, my system is about 20 g so a big change could be arranged, maybe it would help "reset" the system. A refugium or an ats is something I thought about many times but unfortunately the sump is very small and my skimmer (bubble magus a5) takes almost all the space so maybe doable but not easily...I also thought about a sulphur denitrator but all the commercial and diy designs I found are way too big...

 

[Shirak]

20g is very manageable! I would go that route first. Empty the tank and sump and start with fresh salt water. Soak some paper towels in new salt water and you can place them on the corals to help keep them moist while draining and refilling. Can also get a spritzer bottle with new SW and spray them periodically. Get out as much of the old water as possible.

 

[Randy Holmes-Farley]

I'm not sure how we got from a kit that seems to be having problems to thinking the tank needs a 100% water change?

 

[Shirak]

I'm not sure how we got from a kit that seems to be having problems to thinking the tank needs a 100% water change?

OP has tested fresh salt water and kit worked fine. Did 25% water change and saw corresponding drop in nitrate. So seems kit isn't the issue, tank water is way high in nitrates, very low/zero phosphates. Seems with a 20g system best next step is big water change to get nitrates down and then go from there.

So big question is how to prevent in the future from getting so out of balance? Dose phosphates? Feed higher phosphate food? Nitrate reduction of some sort? 25%/week water changes were working but that seems like quite a bit more than should be necessary. Tank is bare bottom so maybe need some blocks in the sump if there is room? More skimming or fleece/socks?

 

[Aleph]

Well about the testing the drop was tested with another test kit (Salifert) when nitrates tested at 30-50, with the kit I asked about in this thread I did not do water changes yet, still I did a dilution test in the first post that seems to show that whatever it is that causes precipitation and odd color (even very high nitrates) scales considerably with dilutions. Could still be a false reading, who knows, but as @Shirak pointed out there seems to be nitrate build up anyway so something must be done to prevent that.
@Shirak about the solutions you proposed socks don't change anything, already tried and I have 1l of pond matrix and 1l of siporax in the sump already

 

[Randy Holmes-Farley]

If the nitrate was 500+, that's certainly a concern and warrants a water change.

Before doing that, though, I'd confirm with a different kit.

 

[Aleph]

I'm not sure how we got from a kit that seems to be having problems to thinking the tank needs a 100% water change?

@Randy Holmes-Farley yes things surely got out of hand! If you think the thread is going too much off-topic I can open another one somewhere else...

If the nitrate was 500+, that's certainly a concern and warrants a water change.

Before doing that, though, I'd confirm with a different kit.

Thanks for the suggestion of trying another kit, I will try that for sure!

 

[Aleph]

Ok here is the (bad) tank picture:

There used to be lps and sps but they all slowly died.

I managed to get my hands on a Salifert test, did it today and here is the result:

I also did a tenfold dilution in rodi and got this:

It seems that Salifert gives a result of about 100 ppm.
For reference I also repeated the Aquaforest test with a tenfold dilution and got this:

It seems a bit less than the last time I performed the test but I guess it could also be due to the dilution error (I made the dilutions with a 1ml syringe).

So I guess I will arrange a big water change and then try to figure out some other way to keep nitrates low, thank you so much for all the help!