I'm sorry you are losing so many (at least it's not all at once - that would indicate water quality issues). If I knew the life span, I would tell you, but I'm not sure and I can't find anything on the net for you. I've had your post as a hot topic since I read it the first time, and I will keep it up until you get an answer. I wish I could give you an answer.
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Phishie
Site Coordinator

"An ounce of prevention is worth a pound of cure."