Personally, I flat refuse to use HOB filters on any of my tanks unless all other options are exhausted, and I usually recommend that people consider spending the money to upgrade to a canister. Considering that the cost difference between a decent size HOB and a decent size canister is usually only $100-150 (and sometimes less), I can't see any reason other than extreme budget constraints to choose to buy a HOB filter instead.
Mechanical filtration in HOBs is never going to be as good as a properly designed and installed canister. Because the media chamber in HOTs isn't pressurized, water can bypass the media and be returned to the tank unfiltered. Canister filters cannot do this. Their flow rate may drop, but in a correctly designed pressurized filter 100% of the water will be filtered before it's returned to the tank.
Canisters also tend to be quieter (unless you're running something like the Ocean Clears on my 400 with their fan cooled external pumps, at least), offer a larger volume for media and offer better flexibility with respect to media choices than HOB filters do. They're also less of an eyesore, and they reduce evaporation rates compared to HOB filters if the spray bar is installed correctly (under the water's surface).
Canisters are not the be-all, end-all of filtration; internal filters (of all kinds, including power filters and sponge filters) and UGFs still have applications. Trickle filters and algae scrubbers both have their place, as do UV sterilizers, skimmers and ozone injection. But I can't think of anything that HOB filters do that something else doesn't do better. They're a poor compromise with no redeeming qualities in my eyes other than relative ease of use, size and cost, and they're my absolute last resort when it comes to filtration.
Mechanical filtration in HOBs is never going to be as good as a properly designed and installed canister. Because the media chamber in HOTs isn't pressurized, water can bypass the media and be returned to the tank unfiltered. Canister filters cannot do this. Their flow rate may drop, but in a correctly designed pressurized filter 100% of the water will be filtered before it's returned to the tank.
Canisters also tend to be quieter (unless you're running something like the Ocean Clears on my 400 with their fan cooled external pumps, at least), offer a larger volume for media and offer better flexibility with respect to media choices than HOB filters do. They're also less of an eyesore, and they reduce evaporation rates compared to HOB filters if the spray bar is installed correctly (under the water's surface).
Canisters are not the be-all, end-all of filtration; internal filters (of all kinds, including power filters and sponge filters) and UGFs still have applications. Trickle filters and algae scrubbers both have their place, as do UV sterilizers, skimmers and ozone injection. But I can't think of anything that HOB filters do that something else doesn't do better. They're a poor compromise with no redeeming qualities in my eyes other than relative ease of use, size and cost, and they're my absolute last resort when it comes to filtration.
