I had the same problem a few months ago, I went out and bought a new fish with out testing my water, and my nitrates almost killed the fish before I took it back.
two suggestions to lower your nitrates:
1> Do a series of 20% water changes untill your nitrates read zero, probably over a week or two.
2> Get some Macro algae (ex: cheato) from your LFS. If you have a sump or a fuge you can put the cheato in there, if not put the cheato into your tank. Then do a series of water changes to get the nitrates down to zero.
The only sure way to lower nitrates is to keep up with your water changes, I try to keep my self to a schedule of a small water change once a week, with a larger 20% water change once a month.
As for the Lawnmower Blenny and the green Chromis surviving, if your fish were added before the high nitrates, then the they were in the tank with the steady build up of nitrates, which is why they havn't died. The Second reasion is both the LMB and the Chromis are both hardy fish, which could handle the stress of being put into a tank with higher nitrates. Tangs and angles are more sensitive to water quality, which is why they didn't make it.
Now for the good news. The presence of nitrates is kind of a good thing. That indicates that your ammonia cycle is working properly, and that the bacteria in your tank is breaking down ammonia and giving off nitrates as a byproduct.
-Prk543
