If I were you, I`d forget trying to recreate the wild, although it`s nice that you`re trying to duplicate it for them. Difficult, if not impossible, do it in a 30-gallon tank. I`d go for what would keep the water clean (and therefore help keep them healthy) and what is easy to maintain. That means no sand or tiny gravel; smooth river rocks bigger than their heads are better so they can`t eat them but still have something to push around and forage in (you don`t have to use a lot). Get them some cuttlebone (the stuff for birds), take off the hard backing, break it into pieces and let some float in the tank for them to munch on when they want. I`ve read eating gravel/small¡¡pebbles is a sign a turtle wants calcium, and cuttlebone is a good way for them to get it.
Check your water and basking area (the one you will get really soon, I hope) air temps.--water temp. around 78-80 degrees F (too cold and your turtle will become inactive and susceptible to respiratory infections) and basking area temp. 85-90 degrees F. with a UVB light and heat lamp (or combo mercury vapor UVA/UVB lamp).
The fact that you`ve noticed your turtle has become inactive indicates that something is wrong, and even though you haven`t had any problems before, they can take some time to develop before there`s trouble. It would be better to correct their environment ASAP to nip it in the bud.