Well I'm no pro...but have managed to get some pretty impressive pics...I'd start by lighting up the inside of the tank and don't have the light coming from the room.
Turn your tank light on...room lights off.
Most newer smart phones will take some great pictures..make sure your glass is clean and any pumps are turned off. The movement of the water will make it harder for the camera to focus.
most of the cheaper canons and Nikons use the same sensor and the difference is in the features (which generally aren't so great). get a cheaper canon and buy a decent macro lens and a tripod. should be able to do the whole thing under $1000 if you buy a used lens and tripod from B&H or Adorama.