Steve mentioned that something might be feeding the amonia into your tank. I agree. Another possibility might be that your aquarium is not cycled completely yet. Your aquarium has been setup for for like about 6 weeks. Sometimes, the nitrogen cycle takes a little longer to complete. As far as the bio-load, it is high for a 20 gallon. I would be careful with the anemone, if it dies it could release toxins and kill everything. Some suggestions would be too feed lightly and keep up with the water changes you are making. A skimmer would be very helpful but like mentioned it could be very difficult to adjust. BTW, some reputable reefers have mentioned that Bio-Wheels contribute with nitrates into the aquaria. I am not an expert so I can tell you why, I am pretty sure some one will chime in and explain better. Good luck with your tank and I hope my info can help.
Jerry
Jerry
Trying to get back in the hobby... Will be seting up my 75g rr soon