I took this back in June -- my bet is these guys are getting big fast. Not only was it dark , but the tank was also filthy with cloudy water. This one looked terrible in color, which is why I converted it to B/W. Taken with the 30D and a Sigma 100-300mm f/4. Lens was at 100mm ISO 320. The lens was really too long for this venue. Your 50mm f/1.8 should do great .... set the focus to where the fish will be before it swims by and you'll have a good chance. Here's one of the color images -- you can see why I converted it.