I've got one male betta living in a 1 gallon bowl in my dorm here, and I was writing on my mirror with dry-erase paint pen, and the pen suddenly splashed a lot of paint everywhere, and I'm worried some of it may or may not have gotten into the bowl. What should I do? (Also, forgive me if this is the wrong place to post this, if you want to move it, please do.)
If it is not cycled, I'd definitely do a 100% water change for him. Did you actually see any get near and/or in the tank? I'm not sure what possible side effects might be attributed to getting paint in the tank, but I would assume it would be similar to getting chemicals in/near the tank.
So yes, water change and then watch him for any signs that something might be wrong. If a lot of paint got in the tank (even if it is 'non-toxic', this doesn't usually apply to fish), I'd almost suggest scrapping it and put him in a different tank. This is just my opinion though, I'm sure other people might have different opinions on the matter.
I have put some terrible things in my tanks (by accident of course!). The one I remember the most was testing the ammonia in my 20 gal sorority. For some reason I was stupid and was holding the tube of water and one of the test chemicals over the tank with the open lid. Needless to say, I ended up doing like two 80% changes. No one died, luckily. Just wash everything really, really, really well. If you have live plants, they may or may not absorb the toxins depending on what the toxins are.
And I second the idea of a lid. For my 2 gallon drum bowl I got a sheet of plastic from Osh for like $5 and just cut it to size. It was super easy and now I don't have to worry about the cat getting in or the fish getting out!