I wouldn't use cinder block - I believe they are made of cement and similar materials, and could affect pH/hardness.
regular red clay brick could work, or any sort of silicate stone. For something that massive though, you may want to use something not quite as heavy.
Lots of people have stuffed nylon/mesh bags full of gravel.
Lava rock would probably work pretty well. You may want to add some cheaper (inert) substrate to help fill in the holes, so you don't end up with too much of the expensive stuff funnelling down twixt the rocks.