I just did fished doing a water change 35 gal on 140 gal tank hanna checker was flashing 5.0 after the water change now the input on the checker is reading 4.89
Nitrates should be around 10X phos so if your phos is at .2 and Nitrate around 4 then you aren't too far off. Perhaps time to put new phosgaurd in?
Is this a fish only tank?
That is a lot of larger tangs for a 140g. They produce a lot of waste. Maybe looking into a bigger skimmer or nopox dosing to keep the numbers lower...