Is slate a good media for an aquaponics bed?
I am putting together a media bed that will drain into a Deep Water bed and I have come across some slate.  
I am in a limestone heavy area and this may be the only source that is cheaper than buying bags of permatill or hydroton.
This is the kind you dig out of the ground and is not expanded.  I did a vinegar test and it does not bubble.
Any advice would be appreciated.

might be a little rough on your hands bedding the plants down into it.!

It may not have enough surface pores.  I cannot find lava rock or expanded shale/slate.

I don't think that would be an issue. I've seen plenty of systems that use gravel as a grow base and work fine.
It may be much heavier than the hydrogenated clay pellets, but it still works.
Slate doesn't tend to have pours due to its sedimentary construction, but it does tend to have lots of fissures for the same reason.
But honestly. I personally don't think it would be much fun to work with when planting..

You could try making and adding some biochar.