I didnt read all the previous posts in this thread I'm just posting what technique keeps my tank farely safe...Firstly Even fish stores will tell you they medicate there fish only tanks and not to mix the water.. atleast from most of my experiences talking to the managers or guys who maintain the aquariums. I use an airline with a valve on it..extremily cheap.. I take a tupperwear (cleaned previously with vinigar rinsed heavily) put the fish into the tupperwear(with the water fromthe store) and let water slowly drip fromthe tank into the tupperwear (keep topup water handy for your tank to replace your losses) I let it drip into the bucket till i have 50/50... then I remove 50% of that water and drip another 50% from my tank.. once that is over I put the fish in.. firstly by doing this your water remains fairly close to temp so you wont need to purchase another heater..unless your working on a larger scale. secondly by the time my fish gets into my tank it is already swimming in somewhere like 70% tank water..
and you kill alot of the risk for introducing bad things to your tank.... I've had there meds kill corals.. that was the last time I ever dripped a fish. and especially if the staff is just a floor worker then how would they know if the tank is medicated or whatmeds are currently being used..