You're thinking like most people think, that the bubbles are beneficial to the roots. That's not actually how it works.

The solution will hold an amount of DO depending on the temp. As the DO is used up, by the roots and by organisms living in the soup, it is replenished at the surface, where the solution meets the open air. When the surface of the solution is roiled, it causes oxygen to mix if there is room for any. So the bubbles purpose is to roil the surface constantly.

I've never used an air pump, I just leave a submersible pump running and pointed up at the surface of the reservoir.

HTH

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