I am a paint contractor. I may not know jack about fish, but I know paint. I have done flat too, honestly, your best bet is an eggshell or low sheen enamel. You can roll it on in about 4 coats, and if you don't like it, it peels off the easiest.
You can go to Lowes and tell them you want a Sherwin Williams samples quart. For $5 they will tint whatever color you want and you will have enough for 20 tanks. Hobby store will run you almost that for 1\2 the amount.
I've done black, but consider a very dark blue.
Your tank, just thought I'd throw that in.