Yes they can, I have a few FS's around me that run all their tanks off a huge canister. If they get ich they're screwed.
Actually, in most cases, if they get ich...then the customer is screwed.
awesome tanks! i hope this never happens to anyone but I've had trouble where our power shuts off while were on vacation in the winter, and that leads to a lot of dead fish I HATE WHEN IT HAPPENS!