Fish all died, invertebrates OK, corals seem OK

I talked to some people and my LFS and they thought I should post here for some feedback. I have a five month old BioCube 29 saltwater tank with soft corals and fish which I started July 7th 2014. I have taken my time setting up the tank and have an RODI water system, use Instant Ocean reef salt and have been treating with PhosGuard and Purigen the past few months for some red Cyanobacteria that has been showing up in a few areas of the tank (low flow). On Friday morning I switched out both the PhosGuard and Purigen and replaced just the Purigen with one I had renewed. I followed the directions and washed it out over and over after letting it soak in a diluted Clorox solution. I also used API Water Conditioner to remove any remaining Chlorine. I mixed up a little of the red slime which had just been on the surface of the sand which I had done between water changes.



By noon the seven fish I had died (2 clowns, 3 chromis, flre goby and watchman goby) - that was when my wife went into the basement and saw that none of the fish were swimming. All the invertebrates including my peppermint shrimp are doing well. Also my corals seem to be OK, green star polyps and mushrooms are great, but the Zoanthids look a little stressed.



My test and the LFS stores showed no ammonia, nitrite or nitrate (usually my nitrate is around 15ppm, but I had added some macro algae a week prior). Their test for chlorine showed nothing.



I have since replaced my activated charcoal bag with new, pulled out the Purigen bag, did a 5g water change (I do this weekly) and rearranged my aquascape since it was a good time to do it. I will wait a week to see how everything goes and try a test fish.



I would like to know what did this and how I could prevent it in the future. Also, is there any other actions I should take to insure the tank will be good for future fish.



Thank you,

Kevin