Help with Red Ear Habitat for Elderly Parents

Redditors! I desperately need some help. Temporarily taking care of parents old red ear whose lived his long life is a plastic storage bin. Finally got him a tank with a basking light and he his loving it! Feels. It's way too small though (Max the Turtle is 5"x7). I can't find a easy water change (elderly parents) kit that is big enough so I'm thinking of going with a 20 Gallon Tank. The largest I can get due to space issues.

Does a filter exist that can handle the turtle's water so the water doesn't have to be changed out weekly, monthly or ever?

Does anyone make or sell 20 Gal pre-mades for aquatic turtles?

Thank you so much in advance.