Wow, Clams, ok. With clams you have to monitor Alk, Cal & Mag much closer and will have to dose. Most people don't usually have success with clams until a system is fully mature. Or at least have a lot of experience. When you say "A lot of big clams" what are we talking about? One large clam can suck up a lot of Alk & Cal, "A lot of big clams" can down right suck it down to the levels you were seeing, 5.5>dKH, in a day or two. You have definitely jumped into the deep end right from the beginning. You are going to have to come up with a dosing plan, and quickly, if those clams are going to survive. Getting someone over there and helping is going to be kind of pertinent at this point. I might even go as far as saying you should consider returning or rehoming the clams.
Maybe looking at your full stock list. Fish, Coral, Inverts... everything in the tank right now and their numbers. A full tank shot will help too.