You don't see islands populated with the "little people" every day
Nice setting... VERY nice. Sounds like you did a lot of homework before you built, which is a good thing for you and your Koi.
In response to your questions;
Salt has value in treating a limited number of parasites, relieving symptoms of nitrite poisoning, relieving osmotic stresses under certain circumstances, but is generally used in specific treatments rather than a pondwide prophylaxis. It is also tough on plants if you have any.
Water hardness comes in two parts. Alkaline hardness (Kh) and General Hardness (Gh) For good water health Kh levels should be in th 100-200 range. If your water is on the low side you can buffer it up with products like Crushed Oyster shell, Lithaqua, Coral, incorporated into your filtration. I recently added Lithaqua to my system and I'm very impressed with its performance. Baking Soda is a quick fix for low KH (and low ph), but the other products are slower acting and longer lasting. All of these work by precipitating Carbonates into the water to maintain a healthy equalibrium which stabilizes ph against going too high or too low.
Gh is a measure of total mineral hardness, and where to maintain it depends greatly on the type of Koi you are keeping and the goals you have for them. Softer water works well for some varieties while hard water is better for others.
Ph ranges from 7.0-9.0 are safe for Koi, but once again, different varieties prefer different levels for good color development.
I'll leave it up to the experts to give you better details once we know more about your Koi varieties and your water as is.
Maybe our esteemed Moderator can spin your questions into a new thread so it doesn't get overlooked. Wadda'ya think JN?
