Last summer I started a 30 gal FOWLR
that eventually had 1 Clarkii Clown, 2 YT Damsels and a BSWGoby. I had a HOB
filter with media inserts and a small cheato ball growing under a CFL bulb. I did 5 gal WC
weekly. Despite that my nitrates slowly climbed to between 40-80 ppm
. That meant those 4 fish were producing 8-16 ppm
nitrates weekly and I couldn't keep up.
In the fall I purchased a hob
refugium skimmer combo and I was able to get my nitrates down to about 0.
My filter was a 40 gal
rated filter with carbon and chemi-pure elite and a filter pad (changed weekly). Even with the additional small cheato it couldn't keep up with 4 med sized fish.
Personally I wouldn't have more than 1 or 2 fish with a filter only system unless your willing to do massive water changes weekly.