Hang-on-Back Protein Skimmer: A Guide for Sump-Free Tanks
A hang-on-back protein skimmer mounts on the rim of a marine tank and exports organic waste through foam fractionation without requiring a sump.
What it is
A hang-on-back (HOB) protein skimmer mounts on the rear rim of a marine aquarium and removes organic waste without requiring a sump. It draws water directly from the display, making it a practical choice for all-in-one (AIO), nano, and rimless tanks that lack the plumbing for an in-sump skimmer.
How it works
Like all skimmers it relies on foam fractionation: a pump injects many small bubbles into a reaction chamber, where amphipathic organic molecules collect on the air-water interface. The resulting foam rises and overflows into a collection cup, exporting dissolved organics before they decompose into nitrate and phosphate. Most modern HOB units use a needle-wheel impeller to chop air into fine bubbles, so a separate air pump is not needed and the whole assembly stays compact behind the tank.
Why choose hang-on-back
- No sump or complicated plumbing required
- Suited to AIO, nano, and rimless display tanks
- Easier to install and access than in-sump models
- Provides nutrient export for tanks where a sump is not an option
Mounting considerations
A HOB skimmer hangs over the back glass, so rim thickness and tank height matter; some models are not suitable for rimmed aquariums, while others are designed to sit on the rim without brackets or magnets. Confirming clearance behind the tank before purchase avoids fit problems.
Sizing
Skimmer rating depends on bioload as much as raw volume, because the air-to-water ratio needed for a stable foam head is set by the nutrient concentration in the water. Matching the skimmer to the stocking level, not just the litres, gives reliable performance. A heavily stocked small tank can need a skimmer rated for a larger volume, while a lightly stocked one may be served by a smaller unit, so manufacturer ratings are a starting point rather than an exact rule.
Maintenance
Routine care includes emptying and cleaning the collection cup regularly, inspecting the pump for debris, and checking the air intake for blockages so the bubble supply stays consistent. A clean air line and impeller keep the foam head stable and skimming effective. A new skimmer often produces watery, unstable foam for the first days to weeks as the unit breaks in, so the cup output usually settles down after the initial period.