Instead of building all the small components myself (which would have taken way too long), I tracked down some add-on shields that will take care of things for me.With the sensors I want to add to the system, I'll be needing a few different inputs as well as 5V for most of them. Instead of daisy chaining I went with a Proto Shield from Ada Fruit. It's a nice easy kit to setup, and with some stackable headers, easy to expand. This lets me add components and sensors much easier than originally planned.
The other main thing I want to do with the controller is handle the power systems and lighting. The mechanical timers we're using now aren't all that great. They tend to lose time, and sometimes they don't click on when they should. It's much easier to do that with the controller, but handing all the extra circuitry needed to run multipe relays was going to be a pain. Easy Solution - the Relay Shield! Now I'll be able to run all the time sensitive devices (up to 4) directly from the controller. Just need to find a decent power strip I can hack.
Finally, the code is almost complete to show the Thingspeak feed directly on the site. That'll mean the front page will have current tank statistics! Nerdy, but cool in my book.
In the meantime, you can see the Thingspeak feed here, and all the code is up on my GitHub and Drupal.org