I will have to disagree (politely of course 

) again b/c your circumstances for shooting may be different than others. I agree that glass vs. glass there is no comparison, pro wins in ALL apertures. But we have to think that not everyone has a brightly colored fish, high output bulbs over the tank, the ability to hold a camera steady etc etc etc. With that in mind, when people as SPECIFICALLY what should I buy first to get better pix with my DSLR, I say a flash. This is the broadest solution to the problem of more light. Then of course, if you have the $$ go ftw and get both. Nice trick with the shots, and I feel a little amateur guessing one of them 
 
btw, did you ever get the 600?