I read on a pond forum where somene used these pool solar covers (basically blue bubble wrap that sits directly on the ponds surface) and lost all of his fish. He thinks it was because with the bubble wrap directly on the pond's surface, there was no oxygen exchange and the fish suffocated. So if you plan to use a solar cover, make sure you have a stong air pump to oxygenate the water.
My idea was to keep a small air-pump running and have a couple pool-rings/inner-tubes or some other floating pool toy under the cover so that gas-exchange can still happen and itll hopefully create a green-house effect too