For smaller aquariums sometimes a small filter fits the bill, better than a larger external hang-on or canister filter. In those cases, a filter like the Aqueon Internal E-Series Filters provide a simple, but powerful option.
The small profile design simply attaches to the back of the aquarium and goes to work. The slim profile takes up less space than traditional internal filters (particularly the old school corner filters).
Replaceable filter cartridges make maintenance a snap and slide easily into the filter. A coarse pad and Bioholder grid also help facilitate bacteria growth to enhance biological filtration.
These types of filters are great for if you have a small tank as your main display, are looking for a simple tank for your office and need a simple filter, or even for setting up a temporary quarantine system.
You can’t beat the price either, as they start at around $8 and come in a 3 gallon Mini, a 10 gallon Small, a 20 gallon Medium and a 40 gallon Large version.