Pool Filter FAQs

Not sure which pool or spa filter is right for you? Whether you're replacing an old unit or setting up a new system, this guide covers everything you need to know about pool filter types, sizing, maintenance, and when to replace. Filters are the part most pool owners get wrong, and the right choice saves you money, chemicals, and headaches all season long.

What does a pool or spa filter do?

Your pool filter removes dirt, debris, algae, bacteria, and other contaminants from the water as it circulates through the system. Without a properly working filter, your sanitizers can't keep up, water turns cloudy or green, and your pump works harder than it should. A good filter is the foundation of a clean, low-maintenance pool or spa.

What are the different types of pool filters?

There are three main types of pool and spa filters: sand filters, cartridge filters, and diatomaceous earth (DE) filters. Each works differently and suits different pool setups and maintenance preferences.

Sand Filters

Sand filters use a bed of filter sand to trap particles as water passes through. They're the most common type for residential pools because they're affordable, durable, and easy to maintain. When pressure builds up, you backwash the filter to flush out trapped debris. Sand filters typically filter particles down to 20–40 microns. They don't catch the finest debris, but for most average-sized pools, they do the job reliably for years with minimal effort.

Cartridge Filters

Cartridge filters use a pleated polyester filter element to strain water. They filter down to about 10–15 microns, catching finer particles than sand. There's no backwashing needed, which saves water. You simply remove the cartridge, rinse it clean, and reinstall. Cartridge filters are ideal for pools where water conservation matters, above-ground pools, and smaller in-ground setups. The tradeoff is that cartridges need periodic deep cleaning and eventually require full replacement.

Diatomaceous Earth (DE) Filters

DE filters use a powder made from fossilized algae called diatomaceous earth as the filter medium. It coats internal grids inside the filter housing and traps particles as small as 2–5 microns, making DE filters the most thorough filtration option available. They produce noticeably clearer water and are popular with pool owners who want maximum water quality. DE filters require backwashing like sand filters, plus periodic recharging with fresh DE powder. They cost more upfront and require a bit more maintenance knowledge, but the water clarity payoff is significant.

Zeolite and Alternative Media Filters

Some pool owners upgrade their sand filters by swapping standard filter sand for zeolite or glass media. Zeolite filters down to around 2–5 microns, similar to DE, and also helps trap ammonia compounds that standard sand misses. Glass media lasts longer than sand, filters finer particles, and uses less water during backwashing. These are a practical upgrade if you already have a sand filter and want better performance without buying a whole new unit.

Sand vs. Cartridge vs. DE: Which pool filter is best?

How do I choose the right size pool filter?

Pool filter sizing is based on your pool's water volume and your pump's flow rate. A filter that's undersized for your pump will clog quickly and can't keep up with circulation. A good rule of thumb: your filter's flow rate rating (in gallons per minute) should meet or exceed your pump's output. For most residential pools, choosing a filter slightly larger than the minimum recommended size is smart, as it extends cleaning intervals and puts less strain on the system overall.

How often should I clean my pool filter?

It depends on the filter type and how heavily your pool is used. Generally:

• Sand filters: Backwash when pressure gauge reads 8–10 PSI above the clean starting pressure, typically every 2–4 weeks during swim season.
• Cartridge filters: Rinse every 2–6 weeks. Deep clean with filter cleaner solution every season or when pressure stays elevated after rinsing.
• DE filters: Backwash when pressure rises 8–10 PSI above baseline, then recharge with fresh DE powder. Full teardown and grid cleaning once per season.

High bather loads, nearby trees, algae events, or heavy rain can all shorten cleaning intervals temporarily.

What causes high filter pressure?

High filter pressure (more than 8–10 PSI above your normal clean reading) almost always means the filter is dirty and needs cleaning. Other causes include a closed or partially closed return valve, a clogged impeller in the pump, or an oversized pump pushing water faster than the filter can handle. If pressure stays high after cleaning, inspect the plumbing and pump before assuming the filter is faulty.

Can I use a pool filter for a spa or hot tub?

Spas typically use cartridge filters because of their compact size and low water volume. Most pool-sized sand or DE filters are far too large for hot tub use. If you're running a combo pool and spa setup, your system may share a single filter sized for the pool, which works well as long as the filter is appropriately rated for the combined volume.

How long do pool filters last?

The filter housing (tank) itself can last 10–20+ years with proper care. The media inside is what wears out:

• Filter sand: Replace every 5–7 years as the grains wear smooth and lose filtration ability.
• Cartridge elements: Replace every 1–3 years depending on use and cleaning frequency. Signs it's time: tears, frayed pleats, or pressure that stays high even after a thorough cleaning.
• DE grids: Replace every 5–10 years or if grids crack, tear, or pass DE into the pool.
• Zeolite/glass media: Glass media lasts 7–10+ years. Zeolite typically needs replacement every 4–6 years.

Why is my pool water still cloudy after filtering?

Cloudy water after filtering usually points to one of three things: the filter needs cleaning, the filter media is worn out and no longer catching fine particles, or there's a water chemistry imbalance (most often low sanitizer or high pH) that the filter alone can't fix. Run a water test first. If chemistry checks out and the filter is clean, the media itself may be past its useful life.

Do pool filters work with saltwater pools?

Yes. All three filter types are fully compatible with saltwater chlorine generator systems. Salt systems still produce chlorine and still require the same filtration. The filter choice for a saltwater pool should be based on pool size, maintenance preference, and desired water clarity, the same as any other pool.

Browse AquaDoc's full selection of pool and spa filters

Not sure which filter fits your setup? Contact our team and we'll help you match the right filter to your pool's volume, pump, and maintenance style.