Monthly water changes, flake food daily and tap water, YUP. That's your problem.
I can barely keep my NO3
's under 10 but this is what I do and it works for me anyhow:
*I feed every other day, sometimes every third day - frozen stuff. Try some garlic and let them go 2-3 days without feeding. That should get them to eat.
*Water changes are 15% weekly, RO
For now, I would suggest a 1/3 water change and stop feeding for a couple days. Then, do another 10-15% water change and a light feeding of garlic soaked frozen food. Wait 5-7 days and do a 20% water change. USE RO
. It's cheap. Especially since you're only buying ~5-8 gallons at a time. GOOD LUCK