I have O mossambicus and they do fine without heating in winter here. We have gone as low as -5 degrees C. That would put water temp at about -2 or -3 I would guess. No thermometer in my pond. The fish just don't eat and don't grow.
These have been bred on my farm. From my original buy-in I lost a great deal, because they were raised in heated tanks..... but the survivors proved hardy to the cold and so have the offspring.
My next tank will be painted black to help temps a bit in winter so that the growing season is longer. Might even eventually recycle the water through a solar geyser in time..... during winter...... for further increases. Plenty of sun here .... as I believe is so there too.
Part covering of the pond with a solar blanket is another option I am looking at. I want low-cost solutions in terms of monthly overheads. With this idea reduced water-surface-to-air will probably require increased DO solutions though.
Hope this helps.
I get the larger system up will be trying a tent of insulnet over the FT, breathes so should be ok for DO exchange, sorry about the ads but part way down the page. 
(j/k)
