As a prophylactic methodology for ich (and only ich), tank transfer will result in 100% success rate with healthy fish. I agree only one fish at a time is desirable. Any other prophylactic methodology has a risk (be it chloroquine phosphate, hyposalinity, or copper) because for ich, the first part of the life cycle is highly predictable from a time perspective, whereas the back end of the life cycle is not. The timing of theront emission is highly variable, normally up to 28 days, but depending on strain of cryptocaryon irritans, it has been known to take up to 72 days. Theronts are the vulnerable part of the life cycle and are killed by chloroquine phosphate, copper if it is at a therapeutic level, or hyposalinity if it is a 1.009 SG. An average life cycle appears to be 1 to 2 weeks; however, life cycle durations may range from 6 days to 11 weeks, primarily because of the unpredictability of tomont development (Colorni and Burgess 1997; Dickerson 2006; Yambot 2003). Virtually noone keeps fish in a prophylactic quarantine that long, and few keep it for 35 days which would be the requirement to kill theronts in the normally occuring time frame. Whereas with tank transfer, 12 days and you are 100% certain to eliminate ich (cryptocaryon irritans) This is why we see anecdotal cases where "quarantine" for ich is unsuccessful.
