ya the altered pics look much better like stated above check ure whitebalance. whats your ISO at btw? sticking with 400 imo is good. and the faster shutter speed the better.
if you want the pic darker, increase the shutter speed. even if your tank is super bright, you can darken it with a very fast shutter speed. the slower the shutter speed, the more light gets in and the brighter the pic