Thanks. It's not that I need to add fish each time. Clam require amonia and nitrate, right? Usually, I kept my nitrate/amonia zero, when I add a new clam, which means more amonia is needed, right? It might be okay if I am just having 6 clams, but when it gets up to 10+, I definitely need more fish then what I have now. My tank is not big enough/having enough room to let that many fish to swim all at once. I don't know that make sense or not?