I have done exactly the same thing with MDF, i was given several views on what to paint it with but I just picked one
if your in the states its easy you can use a particular spray paint out there that is recommended unsure on the name though, but in the UK there is non and that paint in the states is not imported
I have researched and no one seems to know of any but my LFS did paint the back of there tanks with just oil based paints, they would not tell me what paint nor did they sell any stupid LFS although they did state any paint that is water proof is okay if left to dry and cure long enough,
I used plasti cote paint, used a primer and then matt black, you can get this paint from B&Q not cheap in my mind (around £8 a can) but it works sticks to MDF, looks great and in a huge range of colours, one thing I did note is the primer dose say harmful to aquatic life
but the black paint I paint over dose not have such warning's so I went for it
now I don't know what the black paint was like sticking to the wood so i did get the primer and used 2 thin coats of primer just enough to cover the wood fully and sanded left that to dry and cure for a week that used up two cans of paint
I then used around 2 cans of black paint in 2 coats leaving 2 and half days between coats and I am now leaving it for a week to cure and air off
I really hope the paint will be okay its not being submerged just water condensing and then dripping down so very small amounts of water in my eyes.
I loose like half an inch of water in a week if that from my tank
hope this helps I got pics and stuff on my journal, link in my sig