Something is not quite right here. If you have any nitrite it is a sign that something is wrong. Did your LFS
give you any numbers to what your nitrite or nitrate was at? Ricordia is one of the more difficult mushrooms to keep some people just don't have much luck with them (including myself) but it is hard to say with out some numbers.
How long have the corals been in the tank? If you just put them in it takes a couple of days for them to settle in.
Do you change the bio media on the filter at all or recently? Also what kind of maintenance do you do on the sponges in the fluval?
If you are going to start investing in coral i would suggest two things for you. 1. Buy your self a good liquid test kit for nitrates. The dip strips just are not very accurate. 2. Start thinking about getting rid of that gravel in exchange for a sand bed.
Gravel collects all sorts of thing that decay or break down and can significantly contribute to nitrate issues if not frequently (once a week) vacuumed. If it were me i would slowly start removing the gravel a little at a time. Maybe a scoop or two a week. Until you have taken out as much of it as you can. Then buy an bag of dry sand and put that in to the desired depth. I would see if you can find a piece of PVC
tube that you can use to pour the sand down into the water through. This helps a bit to reduce the sand storm of adding new sand.
The sand bed will help to provide biological filtration as area for the bacteria to grow. A good white sand bed will also actually reflect a significant portion of light back up which has shown to help corals. Doesn't collect as much garbage is gravel or crushed coral.