A pump pulls water out of the pool through the skimmer, pushes it through a filter that traps the dirt, and sends it back in clean. It runs a few hours a day all season. Without one the water goes cloudy and then green within days — chemicals alone will not keep it clear.

Choosing between them comes down to two numbers, and both of them buy you less work rather than a better swim:

Tank size — 16", 19", 22"

How much sand is inside. A bigger tank holds more dirt before it needs rinsing out, so you backwash less often and the filter works at lower pressure. On a large pool a small tank is the thing you end up regretting.

Pump horsepower — 1 HP, 2 HP

How fast it moves the water. Aim to push the whole pool through the filter roughly twice a day. Bigger pools need more, and so does anything else you plumb in later such as a heater or a pressure-side cleaner.

These are all sand filters, which is the usual choice for an above-ground pool: you rinse the sand clean in place by reversing the flow for a minute or two, rather than buying replacement cartridges, and the sand itself lasts for years.

If you are torn, size up. An oversized filter costs a little more once and saves you backwashing all summer. Already own a pump and filter? Use the skip option in this step.