Consider this... I don't have a QT tank. I recently moved one of my established betta tanks, which had become overgrown with java fern and algae. (Long story, but it was at my sister's house awaiting a safe location in my house.) The tank had one betta and one nerite snail, so I rinsed off the mopani and java ferns as best I could and removed as much algae as I could, and then set up the tank. There was still a substantial amount of algae clinging to the java fern, so I decided to buy an amano shrimp. It promptly vanished and I found an empty shell - so I figured my betta had shrimp dinner while I was at work. A few days later I added a larger amano shrimp and a tiny bristlenose I was going to move into my 180 once the bio cycle got going. Then over the next few days, my betta started looking more and more listless, until I found him hanging head down in the tank, and I found the BN dead - not beaten up, just keeled over and died dead. So I figured there was probably something wrong with the water, and set up a siphon to drain the tank while I added fresh water. As I was investigating, I noticed my betta had all these little white spots all over him. He was dead the following morning because either the bristlenose or the amano shrimp - or both - was infected with ich.