heya, i have a moderately pricey Canon. it takes gorgeous pics. Canon is a real solid brand, and i have experienced working with some of their cheaper models that take pics of about the same quality. which sorta pisses me off LOL. anyway just make sure your digital camera has at least 4 megapixels. which allows for larger pictures and better resolution. and it is from a brand name you know and trust. (sony you will pay more for the name). you will also want to run pics through some digital filter. i prefer photoshop - and depending on the situation, will use auto-contrast or auto-curves. its a one step quick fix to cleaning up the gray tint that most digitals cast on pics.
(called gamma correcting.) that will really bring out the natural vibrant colors of your tank. good luck