"RichToyBox" wrote:

GH is a measure of the calcium and magnesium in the water. A high number is
good for bringing out the black in koi, but is said to cause shimmies
(unwanted black marks) in the red.

KH is a measure of the carbonates and bicarbonates in the water. These are
used by the bacteria that digest the ammonia and nitrites, and as such it
will go down. When the KH gets low, the pH will crash. You can get high GH
and reasonable KH with limestone, or oyster shells, but these are slow
dissolving. You can increase the GH with plaster of paris. To get a good
KH and get it quickly, use baking soda. If you have a high (over 100 ppm)
KH your pH will be solid and their is no reason to run it, except for
information.

Note: some submersed plants (frequently referred to as oxygenators in
this group) will use carbonates and bicarbonates as a carbon source
for photosynthesis. A large biomass of plants can reduce KH in short
order, as can algal blooms. If you keep submersed plants and fish
together, follow RichToyBox's recommendation that you monitor KH;
drastic changes in pH between afternoon and morning may indicate
reduced buffering capacity of your pond's water, something that may be
remedied by amendment with calcium carbonate, sodium bicarbonate, etc.