What killed my fish?

[fmoss]

I have a 25 Gallon (yes, I know that is small) tank in my office. My tank recently developed a good Nitrogen Cycle and I had a bit of an algae problem (brown, green, red) but Phosphates were low .25mg/L. I had a Domino Damsel in the tank for the entire break-in period and he appeared to be in fine health (active and good appetite) despite a little bit of what appeared to be Ich which I was treating with Myxazin. I also had two Cerith Snails in the tank that were making progress against the algae. I decided to increase the load so I added 3 Domino Damsels (all about 1") and 1 Cleaner Shrimp (about 2") long. Everyone seemed very happy and active. The Cleaner molted within the first 24 hours and exhibits all the textbook behaviours (signalling, etc.). the Dominos were active with excellent appetite and exhibited no territoriality (even the incumbant Damsel).
After 24 hours a single fish began to exhibit symptoms -- he became pale. By the next day he was swimming irregularly and I isolated him to a hosiptal tank where he died within an hour. Within two hours another Damsel died in exactly the same manner. The others continued to seem fine and fed on frozen Brine Shrimp quite hardily. By the next morning, they were also dead. The Cleaner Shrimp and the Cerith Snails continue to be fine.
The Water Quality measurements as of today are Temp -- 25C; Salinity -- 1.023; pH -- 8.0; Ammonia 0 mg/L; NO2 -- <0.1 mg/L; NO3 12.5 mg/L; Phosphate .25 mg/L.
Can anyone offer any insight into why my aquarium has turned into a Damsel Death Camp?
Thanks

 

[kris]

I think what you are going to have to do is the proverbial "do-over". For future reference though, don't add anything new while you're medicating a sick fish already in your tank. Sorry about your fish, those domino's are pretty cool.

 

[fmoss]

Exactly how much of a "start over" is required? Do I have to completely take down everything and buy new filter media and substrate? Would a major water change do the trick? How do you go about sterilising a tank?