What you're after is the flow rate of the filter. You want your tank's volume to AT LEAST be turning over six times per hour. Depending on the type of fish you keep, how large they are, and how heavily stocked your tank is, it's common to achieve a flow rate of ten times the tank's volume per hour. I have twelve times turnover on my pond, because I have big fish in there. Also, I have this thing about filtration. But, anyway, shoot for flow rate, not what's marked on the box or what size the filter claims to be for. To do this, you can combine multiple filters in order to achieve a desired rate of flow. For example, I have three canister filters on my 180, and these, combined, achieve a flow rate somewhere around 8 times the tank's volume per hour.