Hi Larry :wave:
Where are you from?
Welcome to the forum, do poke around and see all the great stuff in the world of lasers. Be sure to use the white search bar at the bottom of the page as there is a wealth of information to see here.
Are you going to use the laser to kill parasites that are in the tank? How big is the tank? Are you going to use the laser from outside along the sides of the tank or are you going to put the laser in the water to get really close? Remember to get proper safety laser goggles for the wavelength of your laser is there is going to be tons of reflections and stray beams happening. Enjoy, have fun and most importantly be safe.