HELP - fish dying

Thanks Karcury, I didn't want to seem picky but in this hobby the more info the better! You have a similar set up to mine. O.k. let's try to figure this out. I don't see any Magnesium and low levels can contribute to Corals doing poorly. We can be sure of this, when fish start to die (for no known reason) something is poisoning them. Something is contaminating the tank weather seen or not. I know this hobby gets expensive and sometimes tedious at times but another (although unlikely) factor can be iodine poisoning.
Iodine can be in salt mixes, additives etc. so a test kit is not out of the question. I dose Iodine so I have one. I know most aquarists don't because they aren't dosing but as I stated it can get in your tank in other ways and unless it is used it accumulates to a dangerous level. As pack pointed out, nitrite and nitrate must be checked along with Ammonia. Sorry, I did overlook the sump part of your original post. With all that is going on with the Corals, water parameters could have been thrown off. A real good product (as you try to get a handle on this) is Chemi pure elite. It is far superior to plain carbon and lasts 6 mos. (that is the part I like). It will quickly absorb not only Ammonia but other contaminants and they have added Ferric Oxide to absorb phosphates with no leeching back into the tank (another thing I like). Let us know the results of the tests.

 

hmm not sure what happended to this but i was meant to say

....and if your adamant nothing has contaminated the water - then who knows.

I wouldnt bother with nitrite, its not really lethal.

you have a complete phenomena! Sometimes these things happen and no one knows why

 

OK. I tested for ammonia, and got a .25 reading, instead of the "none" I was hoping for. This was after we did a 30% water change a couple of days ago, so I am assuming it was pretty bad prior to the change.

I'm not sure at what level ammonia becomes dangerous to fish. I know no ammonia is the only good ammonia. We added some Kent's Ammonia Detox to give us some temporary help, and will do another partial water change this weekend after we get some more RO/DI water.

We lost a small clown fish about a month ago - magical dissapearing act - never found her. I'm wondering if that could have contributed to the ammonia problem since she obviously didn't go back to the ocean to find her father (although that's the story I have been using.)

The remainder of the fish seem healthy, and very happy since we added the Detox, so I'm hoping we are headed in the right direction.

Thanks to all who helped! We are really glad to have found this website.