D600 a pretty good camera for Fullframe entry but I would highly recommend a used d700 with a low shutter count if your only taking fish tank photos for half the price maybe 1000 bucks. I would even consider the nikon d5200 at 24mp. I would only go to fullframe if you have the glass or planning to have the glass in the future, the price your looking at a new Full frame lens is about 1000+ at 2.8f and less. You can get some cheap full frame primes under 600 bucks the 50mm 1.4f and 28 1.8f. I shoot with a Nikon D4, D800 and I sold the d600 because it was a pain to ttl everything for studio work.
Sample images from d5200 at 1600iso and up... remember the picture is cropped
http://www.cameralabs.com/reviews/Ni...00/noise.shtml