well Ca for a clam is huge. my clam even though i just got him i know will use alot of Ca. now i'm going to have to adjust my drip system to sute the needs of the clam. i'm trying to keep my Ca ate about 400. and on top of that my tank is being converted to SPS so that's more Ca right there. so to answer you'r question why is just Ca a problem is because it was mostlikely the the deciding factor in its death. IMO it was probly the many diffrent peramiters being WAYYYYY OFFFFF that lead to the demise of the clam. i'm sorry for you'r brother's loss perhaps he should just stick wiht hardy fish hardy softies if work is going to get in the way. things like xenia, mushrooms, star polyps, zooanthids and zoanthids. then he can start thinking of more delecate animals like shrimp sea stars, and perhaps another clam. The BIGEST thing ppl in the hobby stress to newbies is not water test water test water test but read read read. nothing is better then doing you'r Homework on a particular animal. IMO you shouldn't look at an animal at the LFS unless you have atleast some basic knowledge on it.