I still have not decided on a pump but from all of my research, bigger is better as long as you can dial back flow rates. The logic being if you are not running you pump at its max capacity you will extend the life of the pump.
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my cheap tetra dual air pump is really moving the k1 in my sump pretty good. Its 2 gallons worth of k1.
You could put in power heads for circulation too instead of a pump
how did the K1 work out in your sump?