No wonder why your tank doesn't maintain the cycle. The pH has to be neutral 7.0 or above. Your pH is 6.0 then the ammonia produce by the fish is in non toxic form therefore the nitrosomonas can not convert it to nitrite. Also starving the nitrobactor it need to convert to nitrate . Put some crush coral in your sump to increase your pH. Maintaining the pH is very important to the nitrogen cycle.
But I have nitrate....? I thought PH won't stabilize till tank cycles.