Here's the form you need to order a water test from MU Extension:
https://extensiondata.missouri.edu/pub/pdf/miscpubs/mp0962.pdf

Our soils here in Missouri tend to be on the acidic side.
Ag lime will almost always benefit ponds in this part of the country.
Get a test done to be sure.
MFA or any other farm supply co-op will be a source for bulk ag lime.
Back the spreader truck up to the pond and let 'er rip.