There are ONLY three reasons.
1. It’s tough on most plants.
2. Most of the time, in water that is well aerated and cooler than 78-79 degrees Fahrenheit, a combination of Formalin + Malachite Green is more effective anyway.
3. *The BEST reason to avoid it:* It’s hard to get out of the pond with water changes which can be disruptive to the fish, toxic to watershed plants, and may be expensive.
That is might not cure all parasites is NOT a reason to not use Salt in a Koi pond.
– It doesn’t hurt the fish unless you go crazy with it. I very seldom exceed 0.6% and will typically prefer to use Formalin anyway.
– It’s a fact that if you run salt all the time, you will eventually encounter parasites that are increasingly resistant to it. I treat wholesaler’s warehouses and they’ve got some superbugs.
– Salt seldom if ever has any lasting or significant impact on any bacteria in the environment, ESPECIALLY on Koi pathogenic bacteria.
– Iodine in salt is NON TOXIC to Koi. I’m not just saying that. I practice fish medicine. I KNOW this for a fact. Real life stuff.
– The increase in slime production can present an issue on gills but I have only seen THAT much slime, on THAT damaged a gill only a few times in practice.
– People are reading things about “osmosis” and making ridiculous comments about how salt impacts that, and as long as you’re staying at about 0.3 – 0.6% you’re not causing trouble.
– If you publish on the Internet or have a Youtube channel you have an obligation to be correct and comprehensive. If your data ONLY comes from books and the shit you’ve “heard”, staaaahp.
If you’re kneeling over a microscope at the edge of a pond for years, have a degree in an animal science, biology, parasitology or ichthyology, and you have pathology reports stuffing your filing cabinets – feel free to contribute. Otherwise, just be right.
And salt does NOT cure all parasites. But that’s not a viable reason to avoid the use of Salt.