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Mistakes I have made over the years has to do with equipment failure! Not checking that all is functioning as much as I should have! Also acclimation. When I first started my attempts at ceph keeping, I would be so excited I would rush the process resulting in shock and usually death within 24 hours. The slower the better results! Not escapeproofing your lid is another! Ducktape it to death, even if they have never attempted to come out, there always is the fatal first possability, we didn't think they could squeeze through some little hole that was overlooked!
Good points Carol...rushing is probably the biggest cause of ceph death in the aquarium trade...slow and steady wins the race every time!
Take your time to get the right equipment, cultivate a good friendship at your LFS, and listen to what they have to say...never rush into anything, and you will most likely have a positive experience.