I took care of a slider for a customer of mine not too long ago. Aquired much the same way, too, when it was just a little thing. They had had it for twenty years and their son had long since moved on
The aquarium they brought me was its "travelling" setup, a ten gallon. I was told they keep it in a bigger one at home. Anyway, the aquarium gets half filled with water. There is a suction cup floating fake rock thing that gets attached to the aquarium for when the turtle wants to get out of the water and sun itself. The rocks adjust themselves to your water level and can probably be picked up at Petsmart type places. I kept Willie by a sunny window and they used a heat lamp at home. There is a special turtle food I fed Wille. He got ten pellets/day and fed in a separate large bowl. I left him in there about half an hour in the morning and all the pellets were eaten by that time. This bowl needed to have enough water in it so he could submerse himself when eating and regular tap water was OK. This keeps your aquarium cleaner.
Hope this helps. I figured Willie being twenty+ years old..... they must know what they're doing