Many spiders that end up in your pool filter are harmless, but if you happen to see a live one swimming around, don't try to catch it with your hands. It's a good idea to keep your filter running often and do a net sweep of the pool before jumping in.
You might find giant, dead tarantulas in your pool filter if you live in California. They are large, hairy and intimidating-looking but are skittish of humans. If you find one of these spiders, don't touch it with your bare hands because its hair can cause irritation. The ones you find in the pool filter are probably males, because the females tend to live in burrows. Males like to walk around in search of females during the mating season.
These are common spiders to find in your swimming pool or around it. They like to hide in meadows and forests near running water sources. Plants also attract fishing spiders because they like to hide in them and wait for prey. You might see these spiders walking on the surface of the pool water. Tiny hairs on their legs help keep them afloat. Although they can get very big, they are not poisonous to humans. One way to get rid of the fishing spider population around your pool is to use insecticides and cut down some of the vegetation surrounding the pool.
Also called "diving bell spiders," these creatures actually live underwater. They breathe the oxygen trapped inside underwater bubbles, but they surface occasionally to replenish their oxygen bubble. They are mostly found in Europe and parts of Asia. You can sometimes find these spiders inside pools that have not been cleaned in awhile. Usually, water spiders like to reside in ponds and other still waters, as it is easier for them to cling to underwater plants and catch moving prey.
Northern Tree Funnel-Web Spiders
Many swimming pool owners in Australia find funnel-web spiders in their pools at some point. These spiders cannot jump or live in the bottom of a pool for very long, but they can be deadly to people. Chlorine in the pool water eventually kills them after several hours of swimming around underwater. Air bubbles stick to their leg hairs, which helps them breathe for awhile. Funnel-web spiders like moist, humid areas. Heavy storms sometimes push them into pools, which is how they usually end up in the pool filter.