With such a huge tank, it would be well worth insulating it, e.g. by attaching polystyrene sheets to the back and sides. If you had a background, or first painted the back and sides, the polystyrene wouldn't be seen. You might be surprised by how much electricity it will cost to heat such a big tank, not to mention the lighting, especially if your garage isn't heated. I know people who have given up big tanks after the shock of their first winter electricity bill. If you go for a saltwater tank, you could also be spending a lot on RO water.