There has been some good advice on this thread, but it has gotten pretty confused with people talking about different things without making it clear that they were different.
You mentioned NO3 around 25 ppm stably. That number is maybe still high, but hard to interpret without the phosphate readings, have you been measuring PO4 as well?
Reducing NO3 and PO4 together biologically will reduce in approximately the Redfield ratio. After accounting for the differential molecular weights of the 2, the reduction is approximately 10:1 NO3 : PO4. So if your NO3 is 25 ppm and PO4 is 0.1 ppm, and you are using biological export (like chaeto or bacteria/carbon dosing), then using up all of your existing PO4 will only reduce your NO3 by about 1, to 24 ppm. If your PO4 is 1 ppm and you used it all up, it would get your NO3 down to about 15 ppm. Of course these nutrients are always being produced and consumed continuously by many processes, I‘m just giving a simple example to try to demonstrate the point that they are linked.
Anaerobic bacterial denitrification (NO3 → N2) is a completely different process, has nothing to do with nitrifying bacteria that we need to break down ammonia and nitrite, and it requires anaerobic zones which most people don’t intentionally do anymore. It was popular years ago with deep sand beds, mud refugium, and intentional zero-flow zones like deep in MarinePure blocks. There are serious downsides to this approach, including the potential to poison your tank and knock NO3/PO4 out of balance, by getting rid of NO3 but not PO4.
Almost all reef aquaria are organic carbon-limited. That’s why adding carbon by carbon dosing causes a bacterial population to grow quickly, using up the carbon you are adding plus the NO3 and PO4 (organic carbon is the third and largest part of the Redfield ratio). The bacteria are then skimmed out, in additional to being eaten by coral and microfauna. In general it’s safe because the bacteria that grow quickly and use up the nutrients are generally benign in an established system. However, in a system that is new/unstable, or in the middle of an outbreak of something that can grow quickly like cyano, it can cause a nuisance population to grow faster with bad consequences. This is the main reason why you hear a lot of reefers say it works great, and a vocal unhappy minority say it was the worst thing they ever did.
I use chaeto with a strong light (H380) and large refugium, oversized skimmer, and occasional short term use of NOPOX only as needed. My target levels are approx NO3 1 ppm and PO4 0.1 ppm, but I don’t get too crazy about it, and I don’t check very often. This works well for me, and I absolutely would endorse occasional use of NOPOX following the conservative instructions (not overdosing) once your tank is stable. Like everyone has said (including you lol), rapid changes even in the right direction are worse than slow changes.
