Why is this happening?

[martyna]

Set up:
150 gallon
125lbs live rock
protien skimmer
filstar xp3 canister filter(has activated carbon packs inside)
2 power heads
2 heaters (cant remember the wattage, but i think maybe 2x 300 watt)
Now on to whats happening.
Ive had a humu humu in there from after the tank cycled (almost 6-7 months ago)
Ive lost a few other fish, but hes the only one that seems to make it through everything, and not seem to have any problems. Just recently after doing a water test and everything coming out good, I took a sample to one of the better LFS in the areas, and they said everything was spot on, asside from a reading a of 5-10 nitrates(I think it was that, they probably said it was from fish waste, and would go down through water changes, which i usually do once every other week)
I lost a naso tang, not sure why. Went to bed one night, was swimming around, woke up, and he was dead on the bottom. Then i lost a lionfish who just wouldnt eat, no matter what i tryed.
Then i decided to wait, let the tank settle for a few months and try again.
Placed an order on this site for 2 yellow tangs and a porc puffer. After the drip, put them into the tank (i dont have a QT set up, maybe something i should look into?) turned off the lights for a few hours, moved the rock around, and they seemed fine. Very active and eating (frozen krill, silversides, formula 1 cubes, alge strips and flake food- alternating to vary their diet)
Well after 3 weeks, the puffer's eyes turned white, and ended up dying QUICK. So i test the water, everything seems fine. Then a few days later my 2 yellow tangs are getting really pale, and they both die within days.
So here I am, left with 1 single 4'' humu humu in a 150gallon tank, healthy as far as I can tell, swimming, eating, playing, but yet nothing else has been able to survive in the tank?
Any suggestions? I have noticed the humu humu had some aggression towards the naso before it died, but he didnt even seem to notice the other fish that were the most recent additions to the tank

 

[laddy]

Have you added anything into the tank, anykind of treatment? Ich medicine? Copper additive? The nitrates should be undetected, however 5-10 mg/l I don't believe would kill the number of fish you mentioned--I assume amonia and nitrites weren't detected?

 

[martyna]

Originally Posted by Laddy
Have you added anything into the tank, anykind of treatment? Ich medicine? Copper additive? The nitrates should be undetected, however 5-10 mg/l I don't believe would kill the number of fish you mentioned--I assume amonia and nitrites weren't detected?
No, I havent added anything yet, and the only fish in the tank seems to be fine at the moment. My test kit, and the one at the LFS didnt pick up any thing other then the trates

 

[v-lioness]

Is it possible this trigger is harassing everyone to death?
Naso's & porcupine puffers can be tough, lions depending on the type can be tough and could easily stress living with the trigger, it is also easier to place the lion in quarantine and train to frozen foods before placing them in the display tank. But the yellows turning pale sounds like environmental stress to me....
You may want to think about removing the trigger before any new fish are introduced, also think about setting up a quarantine tank for any new additions.
Kaye