Oh yeah! I haven't got any room left on the board, but it could be done with one of the digital pins on the header.
It seems I also forgot about the ATO, but it could be done with the one SSR I have left free.
The whole idea is to have the aquarium run itself because it's pretty time consuming. The only things I wish I knew how to do is measure calcium and conductivity. The conductivity I think I can figure out, but I've yet to find a calcium sensor anywhere.
Other things I want to measure are co2, oxygen, orp. It would also be cool if I could measure ammonia, nitrates and nitrites as well; however, I think I'll need to move up to an Arduino Mega because I'd need more analogue lines.
If anybody knows how we could incorporate any of these into the board I'd love to hear from you.