I'd say start getting RO
water and only feed them once a day. And half what your feeding them.
I have the same problem, by nitrate stays around 15-20pprm. I got told, To remove it, Do 4 consecutive water changes @ 30% and it should bring it right down to 0-5. But after thats done, Gotta find the source.
Also, Go buy a nitrate sponge. I just purchesed one yesterday and put it into my filter. I'll test today and see if i have much of a result.