Black sealant for indoor ponds
 

I've seen above-ground indoor pond/waterfall constructions made entirely
with natural stones (dressed stones for the pond perimeter and rough
stones for the waterfall). The stones were held together by something
the colour of cement (sand mix perhaps), but the pond interior was coated
in a thick black sealer (some type of tar). Ponds are in the 800 to
1200g range.

Can someone identify or direct me to a source of this sealer.

Is there any additives used with a sand mix to make it more fish-safe or
less porous? I'm already familiar with the leeching requirements with
the fresh limestone in concrete products.

TIA
NetMax