fish deaths

hi, i just came back from a vacation and thought i would ask if anyone knows what might be wrong with a friends aquarium. The problem is that fish don't last alot in his aquarium, most die within a week or two. Besides the ones that are already there, he can't seem to add new fish. I highly doubt it's overpopulation, and his corals and inverts are doing great.
- his aquarium is 70 gallons
- hes got a canister filter
- hes got 2 150 watt MH and 4 actinic t5s
- enough live rock
- shallow sand bed
- and the fish that seem to die most are tangs(except for a yellow tang that is currently there), angelfish and other sensitive fish(like butterflyfish)

 

Are his parameters alright? Seems the tangs and angels dying, which are less hardy and more sensetive to poor water quality might be a sign.

 

i agree with siid. as always in cases like this, params are needed to start

 

Exact params? Maintenance on canister filter? Skimmer? Stock list? RO/DI or tap? Amount of rock? Fuge/sump?
List could go on and on...

 

Best to get your friend to fill in this form if possible: ASAP!!!!!!!Please start here.

 

If he's already got a yellow tang in there it could be beating up on the new arrivals. Lots of stories of established yellows getting territorial. I personally don't think the 70gal range is big enough for two tangs.

 

i don't think parameters are a problem since he has coral and inverts, he has a skimmer can't remember what kind, and the wierd thing is that they last a couple weeks then die. Also he uses drinking water(probably RO) for the fish.
The reason I think it's a mystery is because fish don't die in my tank as as much(and they were from aggresive tankmates)while he takes much better care and has much better filtration than i do.

 

thanks for replies though!