Hello James,
It's really hard to say what type of fluid is being heated! In Tom's reply to your last post he wrote "That specific tank does not have a heat exchanger, so there can not be any antifreeze in the solar loop." However I have read about systems that use a tank like the one you have with an external heat exchanger. If that is the case, you most likely have an antifreeze/water mix.
At the same time, it's possible that there is a drain back tank somewhere else in the system (not pictured), in which case you would be heating water directly and draining it from the system at night. Is it possible that there is a smaller tank (say 5-15 gallons) hiding out somewhere between this picture and the collectors?
It might be a good idea to hire a solar hot water system specialist to come to your place and check out the system to see if it's working correctly. If your interested in learning more about solar hot water systems I would recommending reading Bob Ramlow's Book, Solar Water Heating, Comprehensive Guide. We are also offering a Solar Hot Water Heating workshop on August 18th and 19th.
http://store.altenergystore.com/Books-Workshops-Videos/Solar-Hot-Water-Books/Solar-Water-Heating-Comprehensive-Guide/p4343/http://workshops.altenergystore.com/Solar-Water-Heating-From-the-Basics-to-Installation-Sat-Aug-18-Sun-Aug-19/e15/-Chris
