When I had a 29 gallon I ended up dosing about halfway between 1/16th and 1/8th tsp of KH2PO4, in order to prevent green spot algae. So, you might try that. From your description it seems like low CO2, but the drop checker seems to be saying you have 30 ppm or so, which should be right. Now I understand how you hooked up the powerhead, so that seems all right too.

Do you dose KH2PO4 the same time you dose the trace element mix? If so, you might be precipitating out the iron and some of the phosphate.