That little guy can get up to about 14 inches long so I think your best bet is to get a big tank now instead of constantly upgrading. With possible 14 inches of length your going to need a wide footprint so the turtle can turn around easily. Where do you plan on putting the tank? I'd maybe try and have something built for you or grab a 180 gallon since it has a width of 24". But I think a custom build for you would be great since soft shells like to bury their eggs in shallow sandy/pebble bars. You might think I'm crazy but trust me I'm a turtle lover. I have a 7yo RES and I just got her a 210. Spend the money now and you'll save yourself money down the road.