Fish Death continues.... Any solutions....

[ponokareefer]

I think Myka hit the nail on the head with the specific reasons for most of the deaths, and ingrhaul has some great advise as well. 5 months in essentially a "new" system is still pretty young and the bacteria may not have had a chance to fully develop to handle the bio-load appropriately. Even if using the old system water, rock, sand, etc, the new set up will still take quite a while to establish a balance.
As for the mandarin, I got a mated pair from someone that was eating prepared foods for quite some time, but the male just stopped once it hit my tank. The female is still doing well though.