We have 2 water turtles in a 30 gallon aquarium. If you're familiar with turtles at all, you know that they are really effective at dirtying up the water. Without a good filter system, you can expect to clean your tank every 1-2 weeks unless you want your entire living area to smell like garbage.
My husband has always had turtles, and has even hatched them from eggs. When we rescued these water turtles (from a home who didn't want them anymore) over 10 years ago, we spent at least 30-40 minutes each weekend cleaning the tank. About 9 years ago, we broke down and spent $100 for a Rena XP3 canister filter (they are currently about $135 on Amazon), and it was one of the best purchases we have ever made.
This filtration system can handle up to 150 gallons (in case you have a bigger tank). We purchase the small particle filters and activated carbon from Big Al's Aquarium supply store, and spend about $40 a year total.
This filtration system is absolutely the best! I cannot say enough great things about it. We siphon off the bottom of the tank about once a month, and the water stays crystal clear. We clean the hoses 2-3 times a year. After you get things set up for the first time, it doesn't take a long time to replace media (ie. carbon, filter pads) in the canister itself. It makes no noise when running.
If you own water turtles, or have a larger tank with multiple aquatic animals, you need to check out the Rena series of filters.