Do those floating solar powered fountains do any good?

No. One option is to get an airpump and put a pond-sized airstone at
the bottom of the pond. It's not the air bubbles, but rather the
induced air current, that cause the aeration. The surface of the
pond is a big air-exchanger, and bringing the low oxygen water from
the bottom of the pond to the top, where the air is, does the trick.

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