Altum, I hope you don't mind that I cut a paragraph from your post in
another thread to start this one. I'm sorry if this is bad manners,
but the thread had already wandered and I didn't want to stray more.

BTW, people in sof****er areas also add calcium and magnesium to the
fertilizer. My water is hard so I don't worry about it. I doubt it's
too much of a problem in most ponds since everyone tends to add coral
and/or oyster shells to harden the water a little for koi and goldies
anyway.

Well, I have a soft water problem and use ground limestone to
replenish the calcium carbonate in the water to keep the KH up high
enough to keep the pH from crashing. Problem is the calcium won't
dissolve to raise the KH when the pH is higher than 7.8 and mine stays
about 7.8 or up to 8.2 most of the time. The same with shells, coral
and most other forms of calcium carbonate. Consequently I have a KH
of 1 degree, (Converts to 17.8 ppm.) a good bit of the time after a
rain. Most of the calcium that I have tried seems to raise the GH and
the KH remains low, seldom going over 2 or maybe 3 degrees. I have
reached 50 ppm, but seldom see 100 ppm in my pond. Years ago I read
about a pH pill made from plaster of paris on Doc Johnson's site and I
had some success with plaster of paris containing calcium sulfate, but
now most brands of plaster contain silica and I know nothing about
that, but fear it would be harmful since breathing silica isn't good
for people. I found one brand of plaster for hobbyist that states it
is non toxic on the label and I assume if it contained silica would be
required to carry the breathing warning. I've mixed a few cupcake
sized pills, but haven't used them yet.

What form of calcium would you recommend, and where can I get it?
My GH usually reads acceptable, so I am not as concerned about
magnesium, although I don't know how much is in the water or what
gives me the GH, but I have some Epsom Salts and I suppose that will
suffice if I need to add it.

Regards,

Hal