MLF wrote:


"mark Bannister" wrote
I'm pretty sure nothing can "sink into" fiberglass.


Au contraire, Monsieur. As any boat owner will tell you, the fiberglass
plies are coated with "gel coat", an outer layer of resin not containing
any fibers. This protective layer works pretty well, but breaks down in
sunlight over time, and it can be damaged/scratched by hard objects. Holes
in the fiberglass (as for jets) are usually unprotected by gel as well. I
would imagine that an older tub has lots of wear and more than a few
avenues for "stuff" to get into the layers of fiberglass below the gel.

OTOH, I doubt that the stuff put into a hot tub over the years is likely
to be harmful to the fish.

C'est vrai (two can play at this game!). I would be slightly concerned
about gel coats for boats. I imagine its possible to get them containing
anti-fouling agents - which are typically toxic to invertebrates. I'd want
to avoid those.
--
derek