I noticed that you said you have your oyster garden plumbed in to your 'fudge for the main aquarium; you might have a more consistent flow if you pulled your water from the main aquarium next to it, near or from the bottom of that tank, and then ran it up through your oyster garden, with some sort of ball valve in the middle to control flow, and then a pump running from your drainage tray back to the main aquarium. You could still control your flow, but it would be much more consistent; think of it as a water tower compared to a sole pump.
Only thing you might worry about would be accidentally sucking a fish through and into the clam garden, Finding Nemo style
But I think you could come up with a clever solution to prevent that, seeing as how you designed the thing from scratch
