A 10gl tank for a mature watersnake or garter is small...However it has to grow into the larger tank...and depending on the size aquired...will take years. For example you would not put a n/b king in a ten, though between 10" and 2.5" it makes a good sized home.
Rough greens do quite well in a 10. I have kept many (they are local and numerous here in S. Fla). The trick is to use small branches in the tank so it has lots to climb on and doesn't charge the glass. Snakes do not need to stretch their whole length, so unless you are dealing with a racer type or larger fast moving vine snake, the extra space is not necessary (Nor is the smaller space for that matter confining).
My 23" regal Ringneck does fine in a 2gl tank as it stays mainly under the turf except to hunt and the smaller space makes catching it's fast moving lizard prey easier. A larger tank would hinder it's feeding efficiancy. Bigger is not allways better.
Frank
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"The luxury of not getting involved departed with the last lifeboat Skipper..."