I would definitely get the floating deck. It's worth the money because it works really well and is easy for the turtles to climb up to.
I have two filters. One really good one that takes care of the most of the gunk and then a tiny one that is really efficient at sucking up larger debris (like from the plants). I also got two Florida Flag fish which LOVE algae; they like to suck on the small filter sometimes. I've even seen them eat turtle poop so I think they're really handy to have around.
Definitely agree with filling the tank more... and I suggest buying a small pH stone. They're white and in the shape of a turtle. One stone works for 20 gallons and lasts for about 45 days.