I spoke with Firstfish by PM and discussed a few suggestions.
Firstfish - if there is a member who lives in your area who's willing to come give a look, I'd take him/her up on it. A fresh pair of eyes seeing the tank can make a big difference, as well as running some additional tests, although nitrate has been identified as an issue, it may not be the only issue.
Clowns are generally durable but how many were in the tank at any given time? Even in a big tank they are very territorial, and if you've got more than 2 of the same specie, that can be a problem, and so can having an established pair and then introducing others, especially of a different specie.
What other kinds of fish have you lost?
Could be a compatibility issue as well as a water quality or predator issue.
IMO we're just scratching the surface here.
Jenn