Salt in thermal storage water?

I'm going to put a tank of water circulated through a radiator in my greenhouse for thermal storage. I don't want stuff (mosquitoes in particular) growing in the water, and I don't want it to freeze in the winter. If all the components of my system are aluminum and plastic, could I just put salt in the water?

Yes, BUT..

The phase change of water is a HUGE amount of energy that will keep the rest of the greenhouse from getting a lot colder until ice forms.  It really makes a difference for those mildly cold tolerant crops.

Aluminum + salt = tons of corrosion, eventually turning into a fuzzy white thing leaking water

Salt water still freezes, only at a lower temperature. That's why (if memory serves) the Farenheit scale has 32 deg as the freezing point of fresh water, and Zero deg which is the freezing point of salt (sea) water.

Add enough salt, and it's pretty hard to dispose of that water in a way that doesn't harm life in the biosphere. In the oil industry, very salty "produced water," an unwanted byproduct, must (by law) be re-injected into exhausted oil wells at similar depth.

Glycol is yer friend! (Ethylene, polypropylene or just straight Glycerol (recovered from  biodiesel operations))
Doesn't freeze when your heat source fails.
Toxic to bugs or at least a good entrapment medium.

Just a top coat of oil.... any oil, cooking, motor, vegetable, mineral, whatever your conscience allows will kill and entrap eggs and larvae, skim it every quarter and your golden!

But the peace of mind from glycol is worth the trouble of recovering from a scrapyard.....

Hey Bill, that's interesting. Have you used glycerol in a system like this? Does it prevent a freeze? Does it gel up enough to foul the pumps?

In the 1920-1930 era Glycerol was a common coolant, currently there are a few formulations for heavy equipment use, (Google freeze point of glycerol).

For a small caveat...when I say straight Glycerol I'm not saying raw byproduct of biodiesel leftovers, just citing it as a source of Glycerol.
One of the complications of using Glycerol, is its support of bacteria colonies, so biocidal agents need to be added.
Glycerol is highly miscible a 50/50 mix is protection to -25c and a 70/30 mix -62.

I tried it in a homemade boiler system, running at 200f, and it worked well using a 1/6 hp pump, back then you could get 55 gallon drums of raw byproduct from people experimenting, but now several markets have been established, and it's a commodity rather than a waste byproduct.

Maybe the season for thermal storage and the season for mosquitos don't overlap. Fill it up just before heating is needed, no bugs in the water if it's dry...
Insect screen over openings if for some reason your system needs to be open, or lids/caps/plugs to keep the bugs out (or trapped inside).
Fish? something else that eats mosquitos/larvae?
High flow pumping? maybe grind bugs up? or trap them in-line in a screen?
UV filter like for a hot tub? Other salvaged hot tub equipment could also provide heat enough to prevent freezing/protect your crops, and a pump...