Do aquarium turtles require basking every day?

Question

Sometimes, my turtles are not active at all. They appear to be floating on the surface, completely motionless. I generally change water in that case.

However, I also think that not giving them proper ventilation with fresh air and sunlight could also be making them inactive. I am not sure; is it really an issue? If yes, I will have all of them to bask in shaded light near window every day. If no, how should I handle this situation?

Edit: Added more info.

1. Light source in aquarium - CFL (compact fluorescent lamp) 8 watts, only one lightbulb.
2. Aquarium size is 24 x 36 x 24.5. There is gravel at the bottom of aquarium and some plastic flowers for visual aesthetics. The turtles like to sit on them as well. I don't know how they do, but they can spend hours sitting on them.
3. Filter - two submersible filters. They generally like to sit on top of the filter :). Maybe they want to get close to the CFL attached on top of aquarium. Filters looks like this:

3. One air bubbler - with air stone stripped, so that mosquitoes can't sit on surface and lay eggs.

4. What kind of turtles they are - I don't know, but you can see pictures in my other question here: Red flesh coming out my turtle's tail

Answer 1

Ok, I will leave the filter- and space-questions to others (I am not too good at those when it comes to turtles), but pointers about the light and the sitting on the flowers:

About the light: Turtles do absolutely require UVB-light for their organism to work properly. That means sunlight, or a dedicated basking-lamp. Any bigger pet-store should have them. Make sure you hang it at an appropriate height, so it doesn't get too warm.

About the turtles sitting on the flowers and filter: Turtles need to rest occasionally. In the wilderness, they will be on branches, rocks and similar that rise out of the water. You need to provide these resting-places for them. Turtles (at least the ones you can keep in an aquarium) are not permanent swimmers!

That's the general advice for all aquarium-sized turtles I am aware of at least. To identify the species you have, please take some photos form above, side, and front of your turtle :).