I got this or one like it for my mom and her flower gardens last year... it's pretty cool. It's water resistant in the rain sense, but not the submersion sense. It's cool though because it automatically stops shooting pictures when it gets dark and resumes in the morning.
The best way to do a time lapse these days is to buy a new or used canon digital camera that is on the supported camera list for this: http://chdk.wikia.com/wiki/CHDK
It's alternate firmware for the camera that enables lots of additional features like time lapse, motion trigger, lightening trigger, and more on the cameras. I don't think it's important to be in the tank itself, just in a fixed position outside the tank. A picture every 15-20min over several days would probably be pretty interesting. That also means the likelihood of standing in front of the camera at the time is low.