I have two clams both are doing really well. I do a two part alk and Ca no reactor. They have growen quite fast sense I bought them.

Cheers
Tyler

I have a happy clam. I started out using 2 part additives, but that couldn't keep pace and I've moved to Kalk. (the clam wasn't the only reason for the increased Ca/Alk usage)

As for whether or not you could do it with only water changes....I'm going to say "maybe, but no"

With LARGE enough water changes you might be able to hold your Ca/Alk at acceptable levels...but you would get a steady decline. If the clam lives a while it is going to get larger and demand more Ca/Alk. Eventually, you'll need some sort of Ca/Alk supplementation, so you might as well start there now.