When trying to determine what size canister filter you need for your aquarium, there are a few things to consider. First, you should take into account the size of your tank. Generally speaking, it is recommended that an aquarium be filtered at least four times its volume per hour and up to eight times its volume per hour for heavily stocked tanks. So if you have a 20-gallon tank, then you would want a filter that circulates 80 gallons per hour (GPH) or more. Secondly, consider the types of fish in your tank; some require higher water flow than others so make sure to choose a filter accordingly. Lastly, look at other factors such as available space in your home and cost before making a decision on which model will best fit both your needs and budget! With this information in hand, now you should be able to narrow down the perfect canister filter size for your aquarium setup!