I would recommend a heater for the water. I use one to keep my tub at 70F. If I don't use it, the water hits 63F. Especially with fluros.

Also, how is the tub light proofed? I ran a test on my tub with just water and all the lights. In 6 days it grew slime. I got some of that reflectix bubble wrap insulation stuff--it's like silver bubblewrap. I covered top to bottom. Now my water is perfectly clear and my roots are perfect (while not white they are healty--my nutes stain them).

Good luck with the orphans!