when i cycled my 10 gallon, i used 9 lbs. of fresh (still in the shipping boxes at the fish store) live rock. fully uncured rock has a lot more die off then cured or partially cured rock. by the way, it's basically all partially cured if it's being shipped to you, there will be die off and an ammonia spike. after cycling the tank for like 6 weeks, my phosphates were at .5 with a salifert kit. i used marc weiss phosphate and silicate magnet to get them down. my other tank, i used partially cured live rock and phosphates were at .1 after the cycle. phospate won't harm your current residents, it's the ammonia that's harmful. to your question, 40 lbs. of rock will cause your tank to cycle again. it's quite likely your animals will die. cure your rock in a trashcan and then add would be the best idea. if you were adding smaller amounts at a time you would be ok, but that's quite a bit of rock, even for a 75. hope this helps.
