Pollution of the aquatic environment by sodium fluoride from some industrial processes has been linked to delayed hatching of fish eggs and reduced growth, which is why, when I clean my teeth, I rinse my mouth into the sink, not my Koi pond.
Sodium fluoride is also an ingredient in fluoride toothpaste, but it is not added to tap water. It is a different chemical, hexa-fluoro-silicic acid, which is added to some water supplies.
Either of these two fluorides, I will happily put into my mouth, but what about isopropyl-methyl-phosphoryl fluoride? Certainly not, it is sarin gas. We must not assume that all fluorides are the same.
Natural water sources frequently contain fluorides. These natural levels are sometimes equal to, or higher than, the permitted level of artificially added fluoride. They cannot easily be removed at water treatment plants, so many of us have been drinking, and filling ponds with, naturally fluoridated water, all our lives.
As far as I am aware, there has been no scientific research into whether water that has been fluoridated with hexa-fluoro-silicic acid affects Koi, so it may be helpful to consider the following and see what it tells us.
About 10% of UK water supplies have fluoride artificially added. The only practical way to remove this fluoride is by using the best quality reverse osmosis filters. Few Koi keepers have these filters and I doubt that any of them fill their ponds exclusively with RO water, so it is reasonable to assume that 10% of the nation’s Koi ponds contain artificially fluoridated water. If this water seriously affected the Koi in those ponds, then 10% of Koi keepers would either have to abandon their hobby or go to great trouble to obtain water that does not contain fluoride.
Since this is not the case, it is my opinion that neither naturally, nor artificially, fluoridated water pose a significant threat to Koi.