Yes I used latex paint, though I don't remember if it was flat or glossy. A small container did a 125.
My first tip is to position the tank so that sunlight passes through it. You'll be able to see where you need to apply more paint.
Definitely Use painters tape
Also, it may take more than one session to get the job done.
I used a foam brush - not sure if that's easier or more difficult since that's the only tank I've painted thus far. It took several coats. At first I painted the back like I was painting - left a lot of streaks. It looked cool in it's own right - very "starry night". After a few coats like that, I let it dry. Then round two finished the job. Instead of painting with strokes, I dabbed it on, filling in the streaks. Again, having sunlight pass through the tank will show you the spots you've missed. The streaks provided texture for the paint to grab with the dabbing, which then completely blocked out all light. The result is a deep deep black that makes everything inside POP.
I've also heard of people using spray paint to do the job, which would be easy to do on small tanks, since you can lay them down so the back is facing up. Not really an option for me working on a 125 by myself.
Krylon fusion sticks to plastic and many people use that to paint their plumbing pieces. That's next - painting the XP intake tubes which are blue for whatever reason.