Its a new tank but I transferred my existing reef tank over a period of time. This included live rock, water and eventually corals and fish. The tank has been stable with transferred corals and fish for over 6 weeks. Water changes are every 10 days last changed on Tuesday last week. I dose with Balling lite and No Po X. On the No Po X point I've recently had to replace the return valve as this had seized so possible not dosing when it should have. The filter is a Oase Thermal but at the moment I also have an Eheim Ecco running. The problem started last Thursday when the SPS corals started to show signs of stress. Until that point they had been growing well. For me it has happened quickly and I don't understand why!
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Short term solution - 25% water changes, as frequently as you can manage, even dailyi. Be sure that the replacement water has appropriate parameters. I'd also take out the NoPOX for now since it is not getting the job done. Simplification is good.
Long-term solution - determine source of NO3. I'd suspect your filters. I have no experience with the Oase Pond Filter but this would be my first suspect as its primary purpose given the filter pads in it will be mechanical and biological filtration (NH3->NO2->NO3), also known colloquially as a "nitrate factory". Give it a good cleaning to get sludge and debris out (but obviously don't sterilize it or you will have worse problems than NO3). Eheim Ecco is also fine for mechanical and biological filtration but has very little provision to get rid of the NO3 produced.
If it were my tank, I'd take both filters off line, add a hang on back protein skimmer and probably forego mechanical filtration. Your live rock should be able to handle the biological filtration as long as there is enough of it, particularly since the rock is from an established system.
If you are committed to using an external filter, then get one that has the ability to utilize media that can also perform denitrification (NO3-> N2 gas -> gone). This will take many months to get fully operational, but it is worth getting started now. If you can't (or don't want to) add a sump to your system, then I'd recommend an Eheim canister filter of the appropriate size for your tank. Chuck the Eheim media that comes with it and fill the filter with Siporax (stack carefully to avoid impeding water flow). Since replacing the Eheim Ehfisubstrat Pro medium in my discus tank with Siporax about 6 months ago), my nitrate and algae problems have essentially disappeared. That tank (125g) usually ran at > 50 ppm NO3 with the standard Ehfimech & Ehfisubstrat pro media (in two 2217 filters) and weekly 50% water changes (for more than 18 years). Just adding replacing the Ehfisubstrat Pro medium with Siporax, my NO3 levels are at ~10 ppm with the same weekly 50% water change. A few weeks ago, I emptied both filters (previously only 1 had been running Siporax) and replaced all of the media with Siporax. By the way, one of those 2217 filters is > 18 years old, the other is ~15 years old so any investment you make now is likely to last a long time.
