well if it was do to over feeding,would'nt the ammonia level be on the high side also,or over stocked,i only have 10 fish in a75gal tank,yes maybe some of the fish are on the big side,like sal-fin tang 6 inches in lenght,foxface maybe 6.5 inches,blue hippo tag 5 inches the rest are smaller in size.do 15 gal
water changes every 2 weeks,i'am very religous about the maintenace on the tank,only feed them twice a day 12hrs. apart from feedings and don't go crazy when i do.been there done that.maybe i don't have enough l.r. in tank to handle the bio-load on the system.trust me when i say i don't go cheap on my tank.i ony have about 65lbs. of rock in tank,getting more this week i hope,maybe that will help also.thinking of stepping up water changes to 30 gals a month.just bought a new protein skimmer reef octopus.so i been battling this problem for a while with no ill effects on fish or corals.never goes up 40-60 ppm
stays there all tests at 0ppm except the nitrate level.i will figure it out.