I would suggest the biggest one that the opening in your acrylic will allow. If you have a glass aquarium then I would say just go for the biggest one. I myself bought the mid sized one and once my caulerpa began to grow and it was filled with critters, I only wished I had bought the bigger one. In fact, I just moved everything into a larger tank and the fact that I got to build a bigger refugium was one of the most fun parts! So I say bigger the better (ladies, um, ignore that lie)...