I had the same set up as you when I first found my turtle. 20L and the same rocks built up into a basking area. After a while my tank started to smell even after water changes. I never saw my turtle poop.. When I joined the forum I found out that I shouldn't have rocks in the tank, they an eat them and become impacted. I took the rocks out and found the poop and the reason for the smell. I now use sand and I love it. My husband built me a basking area from egg crate (light diffuser) when I got rid of the rocks. I'll try to find a picture of it for you. I should have started with a Welcome to the forum! You'll find tons of information about keeping your turtles healthy here. And by the way your turtles are adorable!
<a href="http://s1195.photobucket.com/albums/aa400/mjs363/?action=view&current=2010-12-30-1220.jpg" target="_blank"><img src="http://i1195.photobucket.com/albums/aa400/mjs363/2010-12-30-1220.jpg" border="0" alt="Photobucket"></a>

Old 20 gallon with egg crate basking area.
Also both uvb and basking (heat) light should be over the basking area. One turtle looks like his eyes may be a little swollen hard to tell. Just check to see how far away from him while basking the uvb is. Sometimes to strong uvb will cause eyes to swell.
Congratulations on your new additions.
Stupid is as stupid does. - Forest Gump
The only dumb question is one not asked.