I'd vote on flat, but have no experience with this.
Reason for flat, it'll not shine or glare like gloss paints can. Figure the light getting somewhat absorbed would be a more natural background, like your looking into the ocean and it just fades to darker blue in the background.
i don't think it matters as far as appearance is concerned.
i used gloss on the grounds that it would be more likely to weather the salt but i don't know if it is the best route. all i can say is it worked thus far.
It won't matter as far as looks are concerned. Through the glass either will look glossy. As Podman implied, the gloss will pobably clean up a bit easier. I would look for a paint that is "scrubbable" like a good enamel.
Allen
I used Rustoleum on every tank I ever had to paint. It sticks very well, is easily wiped off, doesn't blister and peel (I have a tank that's easily 15yo with the black paint still intact - through all my moves and everything!). Caveat is that you have to use chemicals to clean up, unlike Latex which only requires water.