Getting that really original tank look

I've been searching for ages to create a ying yang garden in my tank.
I've removed all the gravel and found a fish shop that sold pebbles.

That and a bridge, and I have a smattering on the floor, a few big
'uns on top, a bridge crossing over the white to the black pebbles.
All I needed was a chinese pagoda or similar.

Well, two months of searching different shops nationwide, I found what
I was looking for. Unfortunately it was hidden behind the shop I
bought the pebbles at...

Anyway, it's made of clay. The shop keeper tells me that it's fired,
but I suspect that's not going to do the job. Any ideas what I should
treat it with to make it safe for the tank? Shop keeper (a fish
keeper - this was a bonzai tree shop odd eh?) suggested the stuff they
coat pond liners with. Does this sound safe?