I would first suggest getting another filter. I made to make my own since they don't make any good large filters that work for turtle tanks. For smaller tanks, I used Duetto's. You don't want the tank 100% full especially if the heat lamps are on the lid as the turtles could be burnt from the lamps. If you can elevate the lamps, that takes care of that problem. I usually fill tanks about 50-70% full for turtles. My turtle used to bask a lot but the last few months, he's on a partial fast (not eating much) and also not basking much. I recently moved him from a 40 to a 120 gallon tank, and he hasn't once gone up on the new land area. I don't know if part of the reason is that he can't. Your turtles may not be physically able to get up onto the basking platform. Some aren't easy to get on.
Turtles will get tired if they can't rest with their heads out of the water and their feet on something solid. You can create that by building up things under the water for them to stand on. But, you don't want to put in things that easily fall over or that turtles can get stuck in. Turtles would rarely drown from just exhaustion of swimming. They are more apt to drown if they get stuck. My turtle does his best to try to find ways to get stuck but, so far, he's always gotten himself out. I bought some things (don't know the correct word) that are fake rocks. My turtle can rest his feet there. This photo shows the thing I'm talking about on the right.
http://www.fishpondinfo.com/photos/reptiles/tanks/tank40.jpgA new person who joined the forum has 13 turtles! Maybe he can give you some ideas and tips.