I've recently decided to get some new fish, so I took out my yellow tang who has had a serious attitude problem toward new fish.  Exited for the oppurtunity to put some pretty cool fish in the tank now that the yellow tang is out, I added a blue, powder blue, naso, and  purple tang and a red flame angel over about a 2 week period of time.  My plan was to add the fish in such a way that none of them would have time to think they were king of the tank and also keeping in mind not to increase the bio-load to fast. 

All of the fish appeared to be healthy and doing really well, with the exception of the red flame angel who has a bit of a nipped up fin.  Each of the last 3 days I have had one fish die.  First it was the blue tang he was the considerably smaller than the other three tangs, I  probably made a bad decision geting him.  However there was no apparent problems between him and any of the other fish.  Second it was the blue cleaner wrasse who has been in the tank for nearly 3 years, he got along with all of the fish and they loved him to clean them.  Third it was the powder blue tang who also got along well with the others.

All fish were carefully aclimated.   

Since the fish are not dieing all at once and it's happening each day, it almost seems methodical as though there is a preditor in the tank.  Also when the fish have been found dead it has been between 9:00 pm and 8:00 am.  As I type this post I can't help but wonder which fish it will be tonight. 

I will have some water testing done.  I think a hospital or tempoary holding tank should have been used for the new fish when added, incase it was disease. 

Any suggestions would be appreciated.      

Steve,  sorry about all that...

What I'd check for is ammonia/nitrite.  You exchanged one fish for 5, it can take a month or more to cycle sometimes.  The other thing that may be having an effect is dissolved O2.  The corals (really everything alive in the tank) breath during lights out (and light on...but photosynthesis dominates).  The net result is during lights out (at night) a pH drop in the tank occurs due to expelled CO2 and lower O2 levels.  If you have an airpump you might want to run an airstone in the tank or sump during nights until you can eliminate this possible problem.

Hope everything else survives.

Forgot to add... if you don't have an airstone make sure one or more of the powerhead in the tank are pointed slightly upward.  You want to maximize surface area in the tank for proper gas exchange.

Yay!  No dead fish in the tank today.  I think you were right Jon I probably added fish to fast.  I may have been feeding to much also.  I don't have much algae in my tank so I'm trying to get off on the right foot and keep the new tangs healthy by feeding food with veggies in it.  My yellow tang that was in the tank before was quite skinny.  He is now at Seabase in the sea horse pond probably getting plump from the algea. 

Steve,

Since I know your tank I can confidently say, as you already know, that there is no excess nutrient problem. It could easily adjust to 10 new fish.

Is the pH low at 5AM? If so, it could be a problem and ought to be rectified as Jon said. But your coral grow quite well, so I doubt that pH is the problem.

I believe you may have simply had the problem I also experience with new fish. I get excited, caution flies into the wind and I buy too many too fast. I must honestly admit that about one of every three fish I buy, usually dies within the first two weeks. Also, Blue and Powder Blue Tangs are the least hardy of the bunch. The wrasse was happy with the way it was before. It's time was way overdue anyway.

You say that there was not enough algae for the one Yellow Tang to pick at. Then you add FIVE major algae eating fish! I'm sorry, but in my opinion that's not such a good idea in a tank with no algae.  But that's just one opinionated opinion.

one other thing, sometimes fish die for no apparent reason...

I've heard stories of fish shipped in from the philipines that were caught by drifting cyanide through the reef, stunning the fish and scooping them out of the water.  The fish recover, but then die a month or two later.  Don't know if this is true or not, just passing on the rumur :)