I am assuming by clams you mean mussels? If so then I have kept numerous mussels/clams in my tanks over the years with no ill effects and would continue to in future. As long as you keep an eye on your tanks (which I tend to do regardless) and are prepared to take measures to combat any possible problems then 99% of the time you shouldn't have a problem. I have never personally had any of my mussels spawn in a tank and I have kept lots over the years so I am guessing spawnings in captivity happen few and far between or there is some specific condition I have never taken into account. This could only be with Australian species however so I guess taking this into account and researching any prospective species thoroughly would be desirable.