We have built several concrete ponds and one day went around checked
They were at different locations and had different water sources and
were different ages and had different people managing the ponds.
However it seemed that any painting at all of the pond helped -with a
black epoxy paint being the best. For the purist you just need to
make sure that
the paint is "potable water rated." In my opion sealing for leaching
lime is much more simple than sealing for waterproofing.
But do make sure you check the ph and alkalinity of your water source