Here is a schematic of a In-Tank & Hang-On Skimmer which are just what their name implies...they mount to the side of the aquarium and simply hang there! Usually attached via a clamp or draped flange, this method has gained increased popularity with hobbyists that keep fish-only tanks and/or those without illusions of plumbing a sump!
There are two basic variations of the Hang-On Style skimmer; those that feature additional biological filtration options and those that do not. Usually, the models that incorporate additional filtration are larger and have a compartment for bioballs, baling, or other media. Those that do not provide this added benefit are smaller and more compact. Both however, must have their water pumps submerged below the waterline. This has always been my principle objection to non-sump mounted skimmers...the pumps and hoses interfering with the aesthetics of my aquascaping! Many companies are modifying their existing sump products to address that portion of our hobby that are not sump-specific. This is good news for those maintaining or planning simpler installations.
My recommendation on these Hang-On designs is to insure that a drain hose exits into a large-capacity jug or similar receptacle behind the stand. Skimmers that overflow can ruin your day!
