Sandstone and preventing erosion

Hello all,

I'm in the process of building a pond in our courtyard and have a few
questions remaining even after reading all the pool of knowledge stored
in this group (bad pun, I know...)

Basically I need to know how well sandstone will hold up as the stone
used in a waterfall. The waterfall will be about 4 feet high with only
two or three 1 foot "free fall" drops onto basin shaped stones (mini
ponds if you will). The rest of the water will basically trickle down
the surface of the fall.

My concern is if the sandstone will erode and crumble from the water
action. I have a chunk of the stone soaking in a bucket and it is
maintaining its integity, but I can scrape it somewhat easily with a
fingernail. I know everything will erode eventually, but will the
sandstone turn to dust in weeks, months, years or decades?

Would it be prudent to apply a form of penetrating tile sealer to the
stone or would there be a better method ?(can't really change the stone
being used, sorry) I eventually will have plants/fish so toxicity is a
concern but I have plenty of time for all the nasties to leach out if
they ever do.

The pond is about 2000 gallons with an EPDM liner.

Thanks for any advice!
-Bruce