I painted my tank with interior latex about 2 months ago. Just got one of those sample jars from Home Depot, and had it tinted to black. Think it was just over $3, and I used a cheap brush ~$1.
It took several coats, but eventually got it completely blacked out.
I did this before set-up, so I was able to lay the tank on it's face, and had a horizontal surface to work with.
I think you should be able to paint your tank, it will just take a lot of patience. put on a light coat, let it dry, do another light coat, etc.
But yeah, I think spraypaint is easier if that's an option in the future.