Thank you! Is it possible to change too much water? Like if I do up to 80% water change for the next few days everyday to get the nitrates back down to a safe level. Then once it's down start testing to get a better idea of how much and how often to do more changes as a normal routine.This sounds like the reason for increased nitrate, as fish grow water changes need to be increased, what works for 4" juvies, is not sufficient for adults, and adding another fish simply increased bio-load.
You may be at a point with P -bass where a 50-60% water change twice per week or more is needed.
With my tanks in the states, with large cichlids, I had to do 30-40%water changes every other day to keep nitrate at my max goal of 5 ppm.