3/4 is very good and much safer (if budget allows go it). Otherwise, 1/2 will suit your project just fine. For practice, design a board that will entirely use a single sheet of plywood. A 48in X 24in X 24in tanks will consume the entire sheet, the only surplus will be the cutouts for the window and the top. Going 24 inches high will allow you to use 1/2 glass which is considerably cheaper than 3/4 inch (24 inches high seems to be the border for 1/2 to 3/4 inch glass) at least in my area.
A couple of things that I am learning from my project (very similar to yours);
1.) Get the best plywood you can afford. A lot of folks use CDX plywood, yes it is cheap but not the smoothest. Four 5.45 a sheet more (in the 1/2 size) I got birch (25.85 a sheet). Very smooth, easy to work with inside and out. I used fiberglass and gelcoat on the inside (CDX would have been forgiving there), but my exterior aleardy looks like a nice piece of furniture. If I want, all I have to do is put molding to cover the screws and stain / polyurethane the exterior and it will be beautiful.
2.) Go to a good lumber yard and let them cut the wood for you. My shop charges $2.00 for 1st cut and .50 each additional cut. For less than $4.00 I got all the cuts I needed. My tank is square and true.
3.) By using 1/2 or 3/4 birch for the tank, you can use 1/4 birch for the the base/stand. Incorporate some molding / trim and you will have a very nice tank. You may think that making a pretty looking tank is not a big deal (it's just going live in my garage or back porch) but it increases the resale value big time. Building a tank is relatively easy. Basic woodworking skills, very light plumbing and a lot of common sense make a great tank. You'll learn a lot making the first one and you'll soon want to get going on a bigger project. Making a 4x8x2 tank is not much different from making your initial 4x2x2 (No extra glass/acrylic cost). And if the the cost of the glass or acrylic is not an issue, making a 4x8x4 tank is not harder (significant glass/acrylic cost increase).
Good luck and keep us posted.
