I have a Nexia 'home intelligence' system by Schlage that uses z-wave devices. There's also a similar system z-wave system called vera. It lets you monitor things like whether windows/doors are open or closed, (air) temperature and compatible door locks. It's actions are limited to controlling outlets & switches and sending notifications.
You theoretically could do some very basic monitoring with it, but in my experience the communication is not terribly reliable and can be a bit slow. I'm not sure if you could rig appropriate sensors, either.
It may be cheaper than a dedicated reef controller, but it's also much more limited and likely less reliable, so end the end I'm not sure it would be worth it.