Do you have any agitation of the water surface? You may not be getting good oxygen exchange which takes place only at the waters surface. If your return has a nozzle of some kind that can be aimer point it towards the waters surface enough so that you can see some surface agitation. You could also add another powerhead or an airstone to do the same thing. I would also double check to make sure the test kit is accurate and the instructions are being followed correctly, some test kits are fairly hard to read and it can make a big difference if the directions aren't followed 100%.