Turtles are very messy - there is no way around it. You will need to do daily water changes with the current set-up. The little white button in the center of the plastron will go away, but if the water stays dirty (if you don't do your daily water changes) - it will develop a serious fungus very quickly.
With small turtles, with shallow water, I find it is easier to just use a small tank or container that I can dump the water out, and put in new water, without worrying about sand, etc. That can come later - unless if you invest in a quality submersible filter. (If you need one, I have one you can buy, which would be perfect, which would cut down the number of water changes.)
2 x a week is okay ONLY if you have a very good filter - not with what you have now.
Also, please get a full pic of the turtle. I'm curious.