Not very often. I deal with the same lfs 98% of the time, and have not had a problem with their stock. I am really careful when I am buying, checking out the all of the fish in the tank for any sickness, parasites, disease, etc. I know you are supposed to with each purchase, so I am guilty!:evil_lol: There is one time though that I was stupid and bought a plecostumus from W- - - - - t, and of course it had a small case of ick that went unnoticed, and it spread like wildfire. I treated the tank with medication, and it finally went away. I think I lost a couple of fish from it, and learned my lesson! Never again. Now, I only go to this particular lfs, and no exceptions.