I'm building one. but, before I begin, I'm giving you GOOD links. Top notch work. Nothing I'm typing below can help you more that the work of the three links I'm citing. (my content below is what I'm doing, or what I want to do, not how I'm doing... no code...no schematics... the fórums are full of 'intentions', true?)
The best source for help is here: http://forum.arduino.cc/index.php?topic=140740.0 (Automated Reptile Control System) You'll find schematics and sample code too.
The best source for products is here: https://www.atlas-scientific.com/embedded.html (Embedded Electronics for Water Quality Instruments).
There's also a guy down in Brazil: http://reefcorner.org/forum/topic.asp?TOPIC_ID=135383&whichpage=2 (Reef Controller) It's in Portuguese, but you should still be able to figure out what he's doing.
Ok, If you want to read a bunch of history, or about stuff I've bought and things I've proven out, you can read below...But the links above have the real meat.
My Aquarium Controller is actually a derivative of a Comprehenensive Water Quality Monitor and Transmitter that I'm developing. The differentiator is that I'm working on OPC for integration to SCADA systems and MODBus over Serial/Ethernet for PLC integration for ALL sensors. No piecing together multiple protocols, multiple transmitters, multiple topologies, etc.. I make a living piecing together stuff and extracting data from it, but I think it might be nice to offer some of my customers a more integrated solution for some Water Quality Monitoring scenarios. It's simply monitoring and data acquisition, because in industrial/municipal settings, it's better to leverage the hardened controls already in place (my opinión). Obviously, I need some sort of interface for setup/configuration too (local lcd or website). I don't know if anyone will ever want such an all inclusive product, so I might as well make sure I can enjoy it and use it later. Hence the other two uses/derivatives below.
Residential/Consumer use I am expanding it to Monitor and Control of Aquariums. (Using I2C to expand outputs 110AC, 24VDC, PWM 0-10V, 12-24V, etc...) Here, a local interface is desireable (800X480 5" LCD) and/or Web Interface and/or Cel Phone BlueTooth interface. I'll never sell it. But someone else might want to build one like it.
A third application is Remote/Isolated Environmental Monitoring Station. This is where energy conservation, solar, gps and wireless (wifi/celular gsm) connectivity come into play. I'm excited about this one too.
Obviously the HARD stuff is all of the COMMS and Data Acquisition. Admittedly, it's all over my head and I'm quietly struggling with simple sketches for BlueTooth, GSM, Wifi and Ethernet Shields on an UNO at the moment to grasp all of that. Then it's a matter of getting it to work with DUE.
So, when it came to Sensors and I/O. I didn't want to re-invent the Wheel. I did buy the shields from PracticalMaker. His MacroDuino stuff is interesting, but I didn't want to learn another framework. Sticking with pure C or C++ lets me easily burrow ideas from others. (I fancy myself as more of an integrator than a creator....for sure!).
So, I got in touch with Atlas-Scientific out of New York City (www.atlas-scientific.com). Brilliant group of young guys. New Yorkers, so they cut to the chase, but are friendly. They've developed a fair number of integrated circuits (stamps) that basically serve as intelligent transmitters that communicate serially (Rs232/TTL); even producing an inexpensive MUX to really expand the network of instruments. They used them in their own solutions as they are/were doing a fair amount of environmental monitoring for the State and City of New York. Recently, they packaged these up and made them available for sale to the public. Honestly, I think I've bought 1 to 2 (sometimes 4) of everything they sell. Sure, my BOM for my initial prototype is through the roof by purchasing solutions, but that's okay.
So, I'm monitoring Temp, pH, ORP, Conductivity, Salinity, Total Dissolved Solids, Dissolved Oxygen, Color, Flow, Level, Ambient Air, Humidity, Barometric Pressure. Even trying a neat data logger. Power monitoring, power conditioning, etc.. Some of the stuff doesn't serve much of a purpose anymore. For instance the Colorometer was an idea I had for detecting petroleum traces using optics with treated lenses....given up as the science is way, way beyond my capabilities. But hey, I have a few of these sensors now, so they might as well go into my prototype. I'm also including inputs for level switches, spill switches and flow switches. Again all of this has been demoed using an UNO (some even on the DUE), but I've yet to start coding it all as one Project and stick it in one enclosure.
I've assembled everything. Just waiting on a few slick connectors and enclosure to house it. The Aquarium Controller I'm building will NEVER be commercially viable. It's simply too expensive because I rely on too many purchased components and I sense it's going to take two DUEs. It's just the culmination of other Water Quality Monitoring work I'm doing that gives me Access to the pieces/parts. You'll always be further ahead financially purchasing a commercially available product like NeptuneSys's Ápex (pretty decent stuff they are doing on the software side, with the ability to customize the control via a simple structured text command editor) or Aquadyne AquaNode (dated, they haven't substantially improved the product in a decade) . There is an Arduino based controller (angel something), but his website has buggy scripts that lock up Internet Explorer and generate warning messages from my antivirus software.
Wow, you read this far. Sorry, I warned you that there's nothing for me to show. But take a look at the Volume of information contained on the Reptile Blog. Once I start physically building it in the new enclosure I've ordered.... I'm sure I'll get school girl gitty with excitement and start photographing and writing.