Nitrates rising and 0 phosphate..?

[JasonVH]

Mostly LPS reef simple 29 gal setup with only HOB running 4 yrs. No skimmer, sump, algae reactor or the like.

Hanna for nitrate and phosphate (low range not the ULR version).

Lately, nitrates have been reading -+10ish with usually a rise of 1-2 ish between weekly or biweekly 5 gal water changes. Yet the phosphate has been reading 0.00 over the last several weeks.

Is that possible? Phosphate truly bottoming out despite nitrates showing a rise between water changes? Visually the corals look good, showing growth etc. No algae or anything on the rocks besides corals, coralline. The glass does need scraping several times each week(powdery looking algae). Sand is ok, no purple or brown patches.

Reading up on the Hanna phosphate testers(started to wonder if I'd gotten the wrong reagents or ??), some are saying the low range is the usual recommendation for beginners for their first year in the hobby. Once a hobbyist maintains 'stability' they suggest Hanna ULR for basically a 'more finely tuned low range phosphate tester'. Is that a thing or a gimmick to sell more stuff? I freely admit not to understand well enough to tell either way. A bit worried as every one agrees zero phosphate is not a good thing, though.

 

[Randy Holmes-Farley Verified Community Expert]

It's certainly possible and not particularly unusual. I'd personally up the phosphate by feeding more or dosing food grade sodium phosphate, which is cheap and easy.