how much temperature increase can a pond cover give?

so I was looking for a cheap aka electrical free way of keeping the chill of my upcoming pond in the winter, it will help certain subtropical fish stay healther.
In my area the dead of winter water will be in 46-50F range for about 2 months, maybe low to 40's for a week or two.

it seems a simple solution is buy some of those floating solar (radiant heat) pads or do your own with some form of clear insulation.

How much heat can be expected from this set up in winter in southern califonia.. 1 degree increase? 5 degree increase?

ah ok that sounds simple enough, not costly and if I could get 10 degrees out of it then my stock will certainly have no issues overwintering. ty

Duncan6618 said:

Some people use the clear solar pool covers instead of the greenhouse film. It's cheaper than greenhouse film and they say it works just as well.

http://www.solarcovers.com/shop/solar-covers

Just don't float it directly on the water like you would with a pool.

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ah ic looks good. And your advice to not put it directly on the pond, I assume this is so there is still gas exchange for the fish to breath. Or does it give less effect that way? pool cover directly over the water and then pool cover slightly elevated covering the water should give the same effect no?