Clams are beauties, Congrats.
Showing ignorance... Because of theirs size/age, do you anticipate not having to spot feed them so much because of their ability to feed from the water column?
I haven't spot fed them yet. They should get adequate lighting for the clams' zooxanthellae, plus I'm confident there's sufficient phytoplankton floating around in there to sustain them... just thinkin', but always open to suggestions...
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On the sump, I really like the adjustable overflow idea, letting you change the level in the sump by raising or lowering the elbow. With a ball of chatoe in there, will you be putting a sponge over the overflow?