I don't know what you mean by decent size,but I would keep an eye out for any trouble.Chances are he will crawl all over the clam and irritate him.If you have a sump,you can move him there.My "Hermie" has his own tank-he is about 3" without his shell.They can be quite destuctive to coral and other animals in the tank when they get big.
I just ordered some Knopp's KoralVit F from Barry at Clams Direct.It is yeast based and has lots of B complex.It doesn't have to be refrigerated until it is opened.It seems to get good reviews from those that are using it.
