Do I need a protein skimmer?
A skimmer pulls dissolved organic waste out of the water before bacteria break it down into nitrate. It is the most common first upgrade in a reef tank — and also the piece of equipment most often bought for a tank that didn't need one yet.
Do I need a protein skimmer?
Not always. Plenty of lightly-stocked nano tanks run well on water changes alone, and an all-in-one under roughly 60 L with two or three small fish is the usual case where a skimmer earns little. It becomes hard to do without as stocking rises, as feeding goes up, or once you are keeping corals that resent nitrate. If you are changing water weekly to keep nitrate down, a skimmer is doing that job for you.
What size skimmer do I need?
Rated 1.5–3× your system volume is the ideal band. Below 0.75× it is undersized. Manufacturers publish a rating range whose two ends assume opposite stocking levels, so ReefPicker sizes against the midpoint rather than the flattering maximum.
Can a skimmer be too big?
An oversized skimmer on a lightly-stocked tank runs wet and pulls little of value, but that is a far softer problem than an undersized one. The harder limit is physical: in an all-in-one it has to fit the rear chamber, and in a sump it has to sit within its rated water depth and still come out for cleaning.
In-sump or hang-on-back?
In-sump once you have a sump — quieter, tidier, and no bracket hanging off the rim. Hang-on-back suits all-in-one tanks without a sump, and some AIO chambers take a compact in-chamber model instead. Check the footprint against the space before buying either.