Neil, did the livestock perish in those tanks, or is it still alive and active? I'd think that if the livestock is fine, the rock is too. But, having said that, Vazgor makes an excellent point and you never know what the next few months will bring.....
If I were you, and if they'll let you do it, I'd have them replace the rock but keep the old stuff in a circulation tank to see if it comes around.
Who are the "professionals" they want you to consult?
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SeaShell
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