If your GH is low this can be a problem (with a stable pH) during the
hours of the day when photosynthesis is high, even with an adequate KH.
It is possible in a pond with heavy plant growth and/or high algae
growth WITHOUT adequate water hardness (GH), for aquarium chemistry to
become problematic due to increased photosynthesis. So either the
reduction of algae and/or an increase in minerals to aid GH is
important if GH, pH, or KH are problematic.
Products
such as Wonder Shells (or Sea-Lab for larger ponds) can help with
supplying essential mineral cations as well as oxidative stress/poor
osmoregulation, ammonia, & stress from transfer.
Quite bluntly, these are a MUST USE product for any serious pond keeper
Product Resource: Wonder Shells/Sea-Lab; Pond, Aquarium Mineral Blocks, Unique version ONLY available at AAP