Definitely no substrate. It just collects debris and creates a bad
zone at the bottom of your pond.

When you say "goldfish," you're talking about Comets or Shubunkins,
right? I only ask because anything fancier will need to be brought in
for the winter. And definitely, don't just toss pet-store "feeder"
goldfish in there.

Your next question should be "how many fish?" : ) Beyond the "so
many inches per gallon" approach, my advice is to back off that
slightly and leave some room for the future generations. Goldfish are
like rabbits... at least mine are.

I'm not quite sure how spawning will work with your grate... My fish
push right up against the shore when mating--getting into 1/2" of
water or less. I guess yours will adapt to the grate, but you may find
some getting injured against it.

Dave