I personally would not plumb a canister through a drilled hole in the bottom of a tank. It can be done but when I weight the pros/cons... the potential risks outweight any benefits it might bring...
Is the tank glass or acrylic?
I've siliconed a small glass square over the drilled holes in glass tanks several times. It's a quick easy solution to covering the hole and it can be removed at a later time if you wish to use the hole. To do so find a piece of glass that overlaps the hole by at least a half inch and silicone it on the inside of the tank. This piece of glass does not need to be very thick as it's only being asked to span a very small surface.
Another option, and probably the better option for an acrylic tank, is to plut a bulkhead fitting in the hole and cap it. I chose not to take this route on my glas tanks because it would leave a nub of PVC sticking out of the sand.