I have two aquatic turtles. what is the best filtration system bc i have to clean the tank every 5 days?

I have a painted turtle 6 in and a slider 3 inch and they get along great. The slider hasn't grown much.is that normal and how fast are they supposed to grow. Also how often should they eat? I feed the seperatly every day but the older one begs for food throughout the day and now the smaller one is catching on. I bought a filtration system meant for a 40 gallon tank but doesn't seem to catch everything so I have to totally clean it out every 5 days. Is their a better way. Also does the basking light and the filter run up electric bill because mine is double what it was before I got them? Thanks