That is a neat enclosure. I know it sounds counter intuitive given the kinds of habitats you find garters in but what others have said is true. Keeping them in captivity is whole different ball game. It's difficult in all but the largest enclosures, to mimic a wetland habitat without causing or encouraging health problems for the snakes and maintenance hassles for the keeper.

While they do need a large enough water basin to fit themselves into for soaking, they will defecate in it often, especially immediately after you change the water and it will need to be taken out and cleaned often. It's a good idea to provide a damp hide for babies and at shedding time but for the most part think "clean and dry" absorbent substrate and easy, often water changes.

There's plenty of naturalistic options that look nice but also address these issues. Try checking some of them out in the enclosure section of the forum.

If you still plan on getting one or two baby concinnus' they're going to outgrow a 10 gallon in 6 mos or less anyway. They grow extremely fast during their first year (like from 7 inches to two feet in a year or so) and can get quite large.(3-3.5 feet for females) You could use a 20 gallon for a few years but think about perhaps a 55 gallon long size enclosure when full grown if you're keeping more than one. You can keep a few baby concinnus in a 10 gallon to start but like I said they'll outgrow it very fast.

might want to keep that awesome 10 gallon setup for some tropical amphibians instead.