If you can get your tank outside or in a garage with alot of air, I would recommend painting it with spray paint. You can do a forum search for the type of paint others have used to do this.
I used a brush & glopped on (literally glopped on) black super gloss I got from Lowes. I cleaned the back of the tank & globbed it on for the 1st coat... Laying it on as thick as I could (the runs in the paint show on the back, but not on the inside of the tank) then let it dry. That gave the final coat something to adhere to... since it doesnt stick to glass very well. The outside back of the tank is ugly, but no one sees that.
I would have likely used spraypaint, but I wasnt going to lug the 90 outside & back... so I went the cheap & easy route.