Most base element metals such as Aluminium and copper in water are totally
inert and they have no effect on any aquatic life in your pond. Unless
stainless steel is of a really high quality it will eventually rust,
resulting in iron oxide being leaked into your pond. This wont do your fish
much good as it can starve your pond of oxygen if large quantities are
present.

Also remember firstly fish are able to determine movement by reflection of
light on the pond surface and secondly they only have about a three minute
memory span. A heron will remain perfectly still for ages and will only wait
for the fish to re-emerge from their hiding place before catching his next
meal. I would surmise that unless the crate is at least 60cm below the
surface of the pond the heron will probably use it to perch on and gain his
next meal.

Craig

wrote in message
oups.com...
I'd steer clear away from aluminum... from what I know, it can be toxic
to fish... for that matter, I'd stay away from anything metal (except
stainless steel)... If they're smaller fish.... try turning some black
plastic milk crates upside down in the pond (black camouflages better
than other colors), bore large holes in the sides if need-be.... weigh
them down with bricks, or even better... set some pots of aquatic
plants or lotuses/lilies upon them to keep them down. Aside from
that, you could buy a large black plastic stock tub from a
hardware/feed store and manually cut holes in the sides to accomodate
your fishes' comings and goings (and hidings).