I have 2 old eheim canister filters. Has anyone altered any type of canister in a DIY manner to make anything useful? Or does this just go into the "I got suckered by the LFS" pile?
I use mine as a water pump. I have it plugged into a timer and it comes on 8 or 9 times a day giving a nice change to the current patterns in the tank. I run it mostly empty, but sometimes put carbon or a polyfilter in it.
You could use one with a micron filter in it to do water polishing...... I also saw some threads on the DIY section about convering them into a calcium reactor...