Check out Monty's Scavenger Tank, second paragraph under Tank. I don't remember if I used Krylon's Fusion or Rustoleum's equivalent (I don't think there is a lot of difference) but it was one or the other and is made to fuse to plastic. It has been painted since October and is holding up well but does not get rough treatment.
Krylon Fusion has a reputation for being reef safe, and adhering well, so it is great for painting things that will go into the water, but I've wondered if it would be better to look for a paint that I can easily peel or scrape off some day if I want to. I know that some latex paints sometimes peel off in large sections (generally considered a bad thing). It would be worth looking into alternatives if you think you might want to "un-paint" the tank some day.
Do people prefer a flat black, or a gloss black? I've only seen gloss, and it looks a little unnatural, even distracting. Flat black might be much better, or much worse looking; any opinions?
I used krylon fusion on my last build and it worked really well. You can still scrape it off with a razor if you need to. The last tank I tried to scrape had latex paint and it was still a pain so I don't think spray paint is an issue.
If you painted the inside of the tank you could get a matte finish but you would have a hard time cleaning it without scraping off paint.
I don't think there are any removable options for painting acrylic but you could use the plastic film and there is a product out there that's supposed to seal it to your tank (supposedly it works well and is removable). I can't remember the name though.
In the same way that silicone won't adhere well to acrylic, I wonder if interior latex paint won't adhere well to acrylic. I've seen interior latex paint peel off in large sheets from surfaces that, for whatever reason, the paint did not adhere well to. I should try some on a piece of acrylic to see what happens.