An even smaller water dish will help. If you can get one.

Unfortunately basements are a little more humid. Especially if its a little cooler. Have you seen what the humidity level is in the room itself?

And its a tank right? Do you have the top covered with anything? That may be raising your humidity levels also.

I've got my little girl in a tub and she's got the smallest water dish the store had and it keeps the tub at a steady 54%.

Just moving your water dish like you did should help out though.