Today is your lucky day
Sargent, I don't know if you will get this post, although I hope you do. If not, best of luck to you. I hope that someone reads this and can get let you know so I can help. I do not profess to be an expert in the field, in fact I am still learning - a lot!!! In the past year, I have learned one thing...how to fix your problem!
The same thing happened to me when I started my tank. Like, the exact thing. I had my tank crash two different times and shortly after that, I could not keep a single thing alive, except for my corals. :frustrat:
Does this sound familiar? This is what I did - after testing EVERYTHING and driving my LFS NUTS with panicked phone calls, crying and beating my head against a wall. $5,000 later (money I wasted trying more fish, additives, treatments, exorcisms, holy water, etc), I have the most beautiful tank I have ever seen - I am a bit biased. lol.
What I say was for me, and I do not want to discount prior information. Everything mentioned has great merit. I went through all of those things as well.
Realize that something on a baterial level is probably wrong. You are testing OK with your water peramiters, right? OK, here is the deal. Leave it sit for two months. Something probably happened to kill the bateria needed to keep your fish alive. Buy yourself a bottle of Bacter Vital (marcweissco.com) which is used to add bacteria to your tank. Follow the directions EXACTLY on the bottle and it will start your healthy bacteria again. Leave your tank ALONE for one month (keep your light cycle down to minimum to keep your corals alive, about 5 hours a day). Algae will grow a bit and may get unsightly - that is OK, get used to scrubbing off hair algae with a toothbrush. After a month, add a good cleaning crew (blue legs, turbos, etc, a good combo of snails and crabs). They should be OK. I have one snail/crab per gallon of tank. Let them work for another 30 days. Start your water changes again - I did 5 gal every other week in a 125 gal tank. Increase your light time to normal. Let your live rock do its job and the new bacteria will do it's job.
It has been two months now and you will be ready to add a fish, something hardy. It worked for me. Time and new bacteria seemed to be the trick. Do not add more than one fish per two weeks. Best of luck to you. Don't give up, the result is TOTALLY worth the trouble. Let me know how things work out.
-Marco :sfish: