Retaining wall back fill/base rock and pool base rock
The rock under the "stack block" wall is different than the rock behind it. 

The rock under the wall needs to be crushed rock and compacted. In my area around here, we'd use 1 1/2" minus (with fines) or just 3/4" minus. Compaction is important. Tamp it down or drive on it. (Usually the stack block manufacturers recommend a base course of 8" - 12" deep and a width equal to or greater than the blocks.  Read their instructions.)

The rock behind the wall needs to have lots of voids so water can flow through it.  So, we use river rock (or some call drainrock). It's smooth (from tumbling around in a river for a few centuries) and does NOT compact.