The short answer
A skimmer is usually a strong default for medium and larger stocked reef tanks, systems with a sump, heavier feeding, limited surface agitation or owners who value an extra margin for aeration and organic export. It can be optional in lightly stocked nano tanks where frequent water changes, mechanical-media cleaning and deliberate nutrient monitoring are already part of the routine.
Foam fractionation removes some dissolved and particulate organic material before it breaks down, while the air-water interface also supports gas exchange.
It does not remove every pollutant, guarantee low nitrate or phosphate, or compensate for overfeeding and neglected maintenance.
Higher fish load, generous feeding, a sump already designed for it, a closed cabinet, or a desire for redundancy in export and aeration.
Very small system, low livestock load, no compatible chamber, reliable water-change routine and nutrients already difficult to keep above zero.
If you run without a skimmer
Write down the replacement plan: strong surface movement, prompt cleaning of filter floss or socks, measured feeding, consistent water changes and regular nitrate/phosphate observation. Refugia and other export methods may help, but each adds its own space, lighting and maintenance requirements.
Do not buy one merely to chase zero nutrients
The goal is a stable system, not the lowest possible test reading. If nutrients are already very low, adding aggressive export without a defined problem can make control harder. Buy a skimmer for a system role—organic export, aeration, load headroom or maintenance resilience—not because it appears on a generic checklist.
Decision
Use one by default for a stocked sump-based reef unless you have a clear alternative design. Consider running without one in a lightly stocked nano only when you can sustain the replacement maintenance. Unsure? Leave physical space and power capacity for one, then add it if the tank’s measured trend justifies it.