[gholland]

What are your ammonia levels like? If you have nitrites showing up, you may have an ammonia issue too, which would be indicative of overfeeding or insufficient liverock (and the bacteria that live in it). Feeding once a day might be too much... does it eat all that you give it? Make sure you remove any uneaten food and make sure the mantis isn't burying it somewhere. I would add more liverock, probably as rubble either scattered around the tank or in the sump if you have one. That should help your ammonia cycle reach the nitrate end-product. After that, try adding a variety of macro algaes to remove the nitrates. I have a fast growing variety of calerpa and prune it regularly.

I also do 10-15% water changes weekly.