Laser diode's need to be current limited to prevent them frying. If you google for "laser pointer power supply" you may find some tips. Apparently the current required to actually "lase" is relatively close the current at which they die.

Most of the cheap laser pointers I've seen limit current with a resistor which is set for the specific number of batteries in the pointer. I think they also count on the fact that the little watch batteries can't supply much current as well (like the cheap LED keylights).
You may be able to get away with a direct swap to AAs, but that may be a little too much and cause the diode to die early, as the AAs can supply more current if the resistor is too low or as temperature varies.

But, you won't be able to hook it up to the tank directly without a good stable voltage regulator to limit the voltage to the same (or ideally slightly below) as the original batteries, and with minimum voltage ripple.
You could also build a current regulator, but I'm not sure how stable it would have to be effective.

Cheers,
D.