aww its too bad you kicked the little guy out of the 55... you know he hates you for that... jk. anyways, its still a nice setup for his size. I started keeping turtles about 2 years ago and last week I just went though my 6th UVB bulb... they dont last.
how often are you doing the water changes?
heres my 2 cents... I found that water quality was my biggest problem with keeping turtles. they poop a lot and I had river stones just like you do, and the poop collected between the river stones, and I had to do a very thorough vacuuming every week. I later switched to fine sand so that the poop did not get trapped and the filter could move it, but then the filter was not strong enough to get it, so I switched to a much bigger canister filter, mated to an undergravel filter tray and witched back to river stones. poop goes through the stones, and underneath the tray, and into the filter.