Filtration is the single most important accessory category for a map turtle, which is unusual among pets but reflects how sensitive these turtles are to water quality. Turtles are messy, high-waste animals, so the filtration system generally needs to be rated well above the actual water volume to keep pace. Canister filters and other high-capacity systems are popular precisely because turtle tanks overwhelm undersized equipment quickly.
A good filter combines mechanical, biological, and chemical stages to remove debris, process waste, and polish the water. Map turtles fare poorly when ammonia and other pollutants accumulate, so the biological component that converts those compounds is just as important as the visible removal of particles. Sizing the media to the turtle's waste output is what keeps the system stable.
Filtration reduces but does not eliminate the need for water changes, and the two work together. Even an excellent filter benefits from a partial water change schedule that dilutes dissolved waste the filter cannot remove. Treating filtration and routine maintenance as a single program, rather than expecting hardware to do everything, is the reliable path to clean water.