[JennM]

I do use RO/DI water to do the change. My Nitrite levels are OK, so is the PH reading. Ammonia is a little high but I attributed that to the nitrate level. What could be causing the Nitrate level to increase.

All the advice thusfar has been good. I would like to explore specifics...

Please post the actual parameters, as it might shed some light on *why* the inverts died.

pH, Temp, Specific Gravity, Ammonia, Nitrite, Nitrate and Phosphate (if you have a Phosphate kit). This will give us a clearer picture of what is going on. Could be that pH or salinity are out of whack, and that's what started the death chain... the rotting corpse of the tang would definitely throw some ammonia into the mix, then nitrite, and then nitrate. Dead snail (peeewww) would do that too. A combination of dead snails/crabs and a fish could be a huge problem, waiting to happen.

Do you have a deep sand bed? If not, if you have a shallow substrate, do you vacuum it to remove detritus when you water change? I see a lot of instances when the hobbyist skips vacuuming shallow substrate and it becomes a big fish potty with all kinds of nasties in it. Deep sand, while I generally don't recommend it, is a bit more "self maintaining" and shouldn't be disturbed.

ANY ammonia is toxic. Do the water changes suggested, and as well I'd consider adding some Seachem Prime or Kent Ammonia Detox to bind up those toxins. They will still register on your test kits, and they still need to be dealt with, but they will be a bit less harmful to the livestock still in your tank. ANY nitrate is toxic and the addition of Prime or Ammo Detox will help this too. Nitrates under 20 ppm in a reef tank are "ok" but the closer to nil, the better.

Jenn