The O-Rings on the filter's
This is just advice on the bad O-Rings that some of the filters utilize. If you discovered a leak on that canister filter of yours and you can't make it to your LFS or they do not have it in stock, you can simply use Vaseline. You simply put a light coat around the entire O-Ring and simply put it back together. If you own a filter that as a water control valve in the intake tube and it has stopped sucking up water. Remove the control valve and apply a light coat of Vaseline to the O-Ring and reinsert it. It will seal up any cracks in the O-Ring and stop sucking in air which is causing the water not to flow through the tube into your filter. The Vaseline will not harm the fish or contaminate the water. It actually keeps the O-Ring from drying out.