I'm using a Canon EOS 6D with a Canon 100mm f2.8 macro lens which is a bit of an overkill but its part of my photographic assignment gears. You can start off with an entry level DSLR + kit lens + lens extender or a decent point and shoot. Nowadays the higher end smartphones are able to produce some pretty good shots as well and you can download the Snapseed app to process the photos (adjust saturation, color balance, sharpness etc) before posting them up. Best time to take will be at night with all the room lights off and only the aquarium lighting switched on.