I have the two smaller ones, the nano and the next one up, the 1600 lph one, both are quiet IMHO, not totally silent but very quiet nonetheless, certainly within very acceptable tolerence levels if you are sitting quite close to one which I normally do when using the laptop. I would have no problem buying more of them, great piece of kit.
I'd definitely agree with the above posts. I have the nano; very quiet device. Tho to start with I did have problems with some finer leaved plants getting caught in there which created undue noise.
I have a standard Koralia nano and a Koralia 1 and they are silent, if they get noisy just turn it off and back on again, sometimes it happens but very very rarely. I even have the nano running between a rock and a bunch of annubias without any problems.
I just bought a Koralia Evolution Nano, I managed to snap the plastic when attaching it to the magnet though doesn't seem like very strong plastic to me. I'll be getting it replaced. Just a word caution!