Fish dying, inverts ok
 

Any idea what would cause damsels to die, with no apparent signs of problems? Pepperment shrimp (have had for over a year) and CBS are perfectly ok, as are snails.

Quarantine your new additions.

I assume you are refering to any additional fish I decide to add in the future?

What would be a reasonable amount of time to qt?

Quarentine everything that you can. Parasites and problems can hitch a ride in on basically anything. A minimum of two weeks is what is recommended by Robert Fenner in his book The Conscientious Marine Aquarist

yep, only way to be safe, quarantine for 4 weeks to make sure you don't bring ich or worse, marine velvet! It can take as long as 4 weeks before marine velvet develop enough to kill all your fish and 2 weeks would not be enough as the parasite could be on a fish but in small quantity and the life cycle of ich or marine velvet is around 4 weeks.

I beleive that you probably introduced some disease, either internal parasites or bacterial, with the new addition of fish, so you should leave your tank empty of fish for 8 weeks now to make sure what ever is in the tank will die off without a host.

Not knowing your system one can only guess. How is your water recirculated. What I mean by that is do you have a lot of surface agitation. You might have a build up of CO. The addition of extra fish might have pushed it over the limit and killed the weaker fish off.
Just a thought.

I have a decent amount of surface agitation, no clue what is enough tho

Down to 1 fish today. Pretty much a guarantee my last yellow stripe damsel is doomed

how old is your tank??

im also going with oxygen are you running a skimmer??

I'm thinking this is probably disease. I have seen some nice smaller set ups without skimmers that do fine.
Damsels are territorial, so having 8 damsels in a system will be very stressfull on them except in a large system. I'm guessing one or more of the new ones had a disease that you couldn't see, and the stress of the move and 8 damsels together caused all the fish's immune systems to wear down and then fish died. As others have pointed out, quarantining the new fish would have probably prevented the quick deaths of all the fish.
What types of damsels did you have before, and what kind were the new ones? Are they known to live together peacefully? Most damsels don't live together very well. In a pet store, they may keep a lot together when selling them like clownfish, but when you get them in smaller groups, they tend to kill each other off.

Video of said tank

http://www.youtube.com/watch?v=v47xrDY3jMc

Running a tunze nano skimmer. Tank is 20x20x9 and a 10g sump