Can you use medium sized, non holey rocks in a mbuna tank, and will they enjoy them or is it just a waste of space to have them? I have found lots of these in my back yard and really want to use them but not if my fish will not enjoy them, thanks
Large rocks make an aquarium look bigger. If you pile them strategically to provide nooks and caves for them to utilize, they'll enjoy it. Most mbuna don't utilize open water so in many cases, not filling the open areas with rock is a waste of space.
There is a great deal of debate over whether egg crate actually does anything. Many people are of the opinion that it does VERY little, and so long as your rocks aren't stacked on sand(they need to be on the glass) and they are sturdy, there's no reason to worry about them falling.
I don't mean to start ANOTHER egg crate argument in this thread, if you have any questions concerning it, I'd do a search for something along the lines of "egg crate pros cons" and see what you can learn there. There are MANY multi page threads about it.
You likely won't get enough rockwork to keep the fish happy, you'll constantly be battling detritus accumulating ALL OVER the tank, and you won't have any foreground. There's no reason to be nervous about stacking rocks so long as they feel solid.
I have lots of big river rocks in mines and my fish love them. I hardly ever see my fish unless it's feeding time. My fish have dug their holes in between the rocks so make sure you set your rocks first then do the sand otherwise you could have a disaster.