also do a big water change (75 % if possible without disturbing temperature) and gravel syphon under EVERY ornament, it always freaks me out how much crud hides under a rock even when I move it every 2 weeks.
p.s. My nitrate poisoning case (Discus that was floating on its back, on surface for a day before I got back to work) took 3 50% water changes over 3 days to fix (BTW the discus came right after day 2, fine now.... (rookie mistake, doubling population, doubling feeding without increasing PWC's)
intial NO3 reading was 40ppm, next was 40ppm, 3rd was (around )35ppm... now on a 25% daily regime until fish are healthy and NO3 down to 10ppm max, then back to 3x weekly 33% change.
also if you can find funding get a sample tested professionally (or find someone locally that can do it properly (without council links sad but true), it might be a massive imbalance in the water chemistry)