Looking to see how many people might use this.

Going on vacation, on travel to work or even being away for 8hrs can be
a nerve wracking experience. Unless someone checks on your fish every
hour you may not have enough time to solve problems once they start.

By combining off the shelf products and a circuit I designed one can
actually receive a phone call if the tank temp drops or the water is low.

I think a lot of people - especially those with large expensive tanks
loaded with expensive fish - might like this idea.

Once a sensor (along with the small box I designed) is attached to the
outside top edge of the tank and a normal phone (not cordless since they
don't work when the power is out) is augmented with an off the shelf
product one can actually be called with a pre-recorded message telling
you the tank is experiencing problems. Several people can be called -
on any phone. This would include friends and pet shops if you chose.

Using this system one could monitor a tank 24/7. If something happens
someone can respond quickly. Maybe a heater needs replaced, maybe the
tank is leaking and maybe the power is out forcing you to have someone
take your fish to the pet shop to house? In any case you are covered.