i had 3 in my 125g for several months. i am now back down to 1. the strange thing is that the one that is left is the original one i got. i think that my tank can only support one conch. all three were showing good growth on the shells so i think that the conditions were good. the one i have has doubled in size. the only thing i can think of is the other two starved. they were a lot more active than the big one, which seems to be an indication that they are not getting enough to eat (purely quessing and observation on my part). they seem to sit and digest for a while after a big meal.
the one i still have did a lot more climbing than the other two did. i will sometimes see it completely up the glass near the surface! i also never saw the one that is left bury itself.
ok enough of the history lesson.
that is a great deal on them. i wish i could put more in the tank. i think they are really interesting to watch. my gut feeling says that you should put one in the 75 and spread the others around. they do like cyano so you could move them around accordingly.
man the longer i am around the longer my posts get.
look out Tom! now if i can only put as many really cool words into my posts.
G~
BTW i have tried 2 conchs and 1 died, he was in there for about 2 weeks and never came out. My thoughts are that he was afraid of my sargassum trigger. I don't know if he starved himself or what. The other is a very small 
She was in my 75g so maybe she was afraid of the trigger to.
