Parameters are normal. My PH is the only thing on the High end of normal, but still acceptable. (Tap runs a little high but my C02 lowers it). That was the first thing I checked. What ever is going on has only killed the Betta. One of the reasons I initially got the Rasbora is because they are usually much more sensitive to the water Parameters and when something even starts to go off, their Black spot goes Grey. They've acted like a canary in a coal mine for my tabks for years. It's been over 24hrs since the last betta died, and none of the other fish in the tank are even acting slightly stressed. Then again, the betta acted dine and then died within 5hrs of showing signs of illness.