How big were the Tangs? How many fish were in the tank prior to adding these ones? How quickly did this crash happen after adding these new fish?

So sorry to hear about your losses :sad:

I have had a lot higher nitrates then 20 before in my tank, did some water changes right away once I realized, and luckily I didn't loose anything. The nitrates probably spiked after the fish died, doesn't seem likely to be the cause of death IMO.

Did you add all these fish at the same time? A sudden death like you describe is likely caused by oxygen starvation. Do you have power heads operating near the surface of your water to allow for oxygen exchange?

^ Ya he did. Although that is a significant amount of fish to add at once, the tank is 160 gallons. I don't think that adding all the fish at once would be the only reason for the crash, although it may have been a significant attributing factor to a snowball effect.

sorry to hear,
did you have a black cucumber?
same thing happened to me long time ago when black cucumber died.
whiped the whole tank out.
Re overflow i have CPR-90 replica brand new newer used for $50

Actually, nitrITE in marine aquaria is relatively NON-toxic to fish:
http://www.reefkeeping.com/issues/20...hf/index.php#5

What is concerning is the presence of nitrITE in the water at all which almost always means that ammonia (which is very toxic) was there previously. There should be zero nitrITE in a tank more than a year old unless there was a mini-cycle - possibly brought upon by the addition of so many fish at once.

I believe that the oxygen deprivation theory may be correct especially if the FX5 was the only form of circulation: Lots of fish are added to a tank with poor circulation. This causes the death of the weakest fish which in turn causes a mini-cycle which kills off the rest. When you finally do the testing the cycle is at the stage where's there's a little nitrITE left and rising nitrATES.

Your Alk reading is actually normal.

^ I think fkshiu is on the right track. I don't think it was a single issue that caused the death of all the fish, but in the end I do believe the primary cause was adding too many fish at once...then the resulting lack of oxygen and (likely) ammonia, and who knows what else may have also played a factor.

oxygen
 

I would be willing to place a large bet on low oxygen levels. Buildup of protein on the surface...usually happens when someone is not using a sump, and an inadequate hang on skimmer (i dont know what skimmer you have/had), and little to no powerheads pointed up at the surface. Did you have a slimy coating on the surface of your aquarium?

Nate

However this is not caused directly by the adding of fish, it would happen eventually if you just had 1 fish. But the adding of new fish increased your need for available oxygen, with that, the new fish are stressed and their oxygen uptake is thus increased, and in the end it comes to a point where no one can breathe and they die. Good thing is your corals should be fine, I have seen them handle this no problem

I'm so sorry about your losses. This really sucks to hear about it. However you will learn from it, and be a better hobbyist after the shock is over. I agree with the posts about oxygen starvation and would look into that. When I first started out I inherited a tank with fish when I bought this house. It had a Ehiem filter and it stopped pumping one night and killed most of my fish. We still have no clue why it shut down since it was clean inside.

So sorry about Alfred.

What kind of skimmer? and most important, did you spray or use any chemical cleaners in your home? even smoke? theskimmer can inject the residuals from the air into your water cusing just what you have experienced including your PH drop. And at 7.6 that alone can kill off everything.