To make a long story short: I recently added a yellow tang and blue hippo tang to my 75. Both fish were relatively small and were doing well. I also happen to have live rock, soft corals, and several smaller reef fish. I decided to add Selcon to the tank as I heard that it worked well in preventing HLLE in tangs. While I was doing this I started to use filtered tap water to top off after evaporation. The filter is the cheepest one out there but seems to make RO water. I brought the top off water and sea water in to get checked and it looked great per the local fish store. SO, to the crash, I added the Selcon and some top off water and that day the tank crashed (6 fish died including one yellow tang, 2 serpent star fish, snails, one soft coral & the rest of them closed off). I did about a 25 gallon water change & would like to add tangs again but I am afraid of this happening again as the reason is not yet uncovered. I have 5 other fish that are doing well that made it through the crash. I heard the selcon could have been rancid but I am not sure if that is it. Any thoughts?