try washing sand in a pillow case or large sealed bed sheet. this holds the sand but water runs right through without losing your sand. :thumb: do this over a tub to see water color. when its clear you should be ok. :thumb:
Personally I would use an indoor latex paint and roll it on. Thats what I used but it took about 3 coats to fully cover. Latex is much easier to clean from your roller and if you wanted to change the color later down the road it would be easier to clean off the glass. Also you won't have the overspray as you would from spray paint.
I masked the trim off with tape and rolled the back and sides with black latex paint. Used one of those cheapo one time use mini-rollers. Came out fantastic; very flat, no streaks. The latex paint comes off very easy with a razor blade, too.
Also if you are doing it before the tank is filled up ( I hope so ) turn the tank upside down so no paint gets inside and mask the trim off like others have said. Some craft stores do sell paint that etches into glass but I never used it and it is expensive for the size of the cans I have seen.
What ever method I did I made sure to layer primer on as a 'FINAL' coat.
That way you have the paint as the first few coats and then the primer is on the outer most coats protecting the paint from being scratched off.