I think putting valves on overflow/drain lines is insane myself. The amount of water that would drain down any one that is properly designed should be miniscule, and obviously a sump should be able to handle it (in the case of a power-outage, etc...). I've seen people make the mistake of closing drain lines and having tanks overflow, or a partly-closed valve just allows opportunity for stuff to clog it. Risky IMO. The only exception is something like a herbie or beananimal overflow... but in those cases you have unobstructed back-up drains.
Just my 2 cents,
Cheers,
Chris
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No more tanks  - Laying off the ReefCrack for awhile!
Cheers,
Chris
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