australia
Fish Addict
i am setting up external canister filter(s) for my tanks, im wondering can i use 1 canister filter for 2 tanks? 


You would have to bridge the two tanks with a piece of tubing. This means that they would end up having the same water level. Two tanks next to each other works out well for this, I've seen setups with 5 or 6 ten gallon tanks on a rack filterd by one canister filter.
You are basically using the tubing as a siphon, to transfer water from one tank to the next. Water seeks its own level, this would make the water levels identical. You then place the intake in one tank, the output in another. Water flows through the bridge from one tank to another.
If the tanks are on different levels this will not work, for tanks on different levels you need overflows with a sump filter. This is what many shops use, I have yet to see a shop use canisters, they are either sumps or air driven filters, such as sponge, box, or ugf.