I have never used any sort of foam in my tanks. I have used the light diffuser (or eggcrate as it's often called around here - don't ask me, not sure how they came up with that) in several set ups before. I mostly just use the diffuser if i am planning on having an aquascape with large rocks or lots of rocks. When I was doing the research I was really surprised on the amount of weight a normal tank could handle, but decided to go with the diffuser just to be safe. Basically the idea behind it is to distribute the weight of the rocks so you don't have so much of a pressure point beneath each rock. Is it necessary? I can't say if it is or isn't, but if you plan to use a lot of rock, what does it hurt? I've used the light diffuser in several tanks with good results, but i haven't specifically seen anybody using foam.