Nitrate spike problems
 

I recently have been having issues with my 50G reef. I was gone for 2 weeks and a friend was taking care of my tank . When I got home my kohl tang was looking pale and just wasn't right. He was the first one. The next day my bi-color died. The day after that my clowns and my flame angel died. I also noticed my watchman goby had been missing. He was the last introduced to the tank. All of my corals, which are mostly LPS we're doing great. The only fishes that survived were my melanarus wrasse and my six line wrasse. I tested the water after my angel died and my nitrates were around the 30-40 ppm range. I initially did a 15 gallon change and tested the next day and nitrates were around the 5-10 ppm. I then did a 5 gallon change and it's been 2 days since I've done anything and it's still around the 10 ppm range. All of the passed fish were removed with the exception of the watchman, which I believe my cleanup crew ate.
I have heard that the introduction of even the smallest fish, such as my 1" goby can be cause for all of my other fish to die. I'm really struggling with this and have thought myself to be an expert as I have been in the hobby for over 10 years. I haven't lost anything in a long time before this series of casualties. Does anyone have any experiences with this issue? I may be missing something here.

I'm not sure exactly what happened, but an ammonia spike due to overfeeding could cause a lot of damage. Were there any recent fish additions before the trip?

The watchman was added about 3 weeks ago and the kohl tang was added about 2 weeks before that

Well, it'd hard to be sure about what happened, then. That might be an illness problem. Were there any other symptoms other than paleness?

I noticed the tang looked like it may have been stung by my bubble tip based on research but I can't be sure. My six line had a pale white spot on its side but is now gone. I didn't see much else with the other fish that passed

From my experience, any time you see an area of pigment loss, suspect marine velvet and treat for it. A heavy bio-load like you have would allow for very quick spread of a disease like marine velvet. You don't always get spots on a fish from this disease. Like ich it often attacks the gills first and the fish die without a mark on them.

I just wanted to comment on something that Bertoni said that may shed some light on what happened. A potential ammonia spike while you were gone. Having someone feed your fish would be my first suspicion. My guess would be that your friend didnt want to starve your fish so maybe just threw in a little extra to make sure everyone got fed. You said you tested nitrates, did you check ammonia and nitrite too? BTW, sorry to hear about the loss : (

I agree with both these posts. At this point, I'd be tempted to try to catch the fish and treat them for velvet and other diseases. It's quite possible that a large excess of food got into the system, as well. I moved to auto-feeders for vacations for this reason.