PHYLAL said:
I am not a big advocate of adding any chemicals to my pond. However many ponders swear by it (others swear at it). PONDCARE recommends 1¼ cups per 100 U.S. gallons for ponds with plants. This should give around .1% to .15%. If you do add salt, remember, it does not evaporate, so when replacing water you do not need to add more salt. Only add more salt if you do water changes.
Trapdoor snails are fine. They are nice to have and do eat algae and some other organic materials. They are live-bearers and will not over populate the pond. However, their cleaning ability is dubious; you would need a lot of them.
I don't think one can equate salt as a "chemical" in the same sense as some off the shelf liquid in a bottle that is a synthetic. I agree with you from that perspective, and I try to avoid adding non-natural chemicals to my pond. However, salt is natural and is normal in many aquatic settings in some form. Our fish are in a "captured" environment, and if koi were kept in completely neutral (basically distilled) water, they would die. Salt is also a wonderful cure for lots of ills that affect our koi. I would certainly rather use salt to help out my koi rather than other "meds" that are used. Many, many organisms have salt in them as a normal part of their makeup, including humans.
I agree with you about snails. I am not a fan of them in pond water. Snails don't eat fish poop, which is what lots of people think. If you buy a good quality floating koi pellet and you feed it in small amount (and only what your fish can eat in 5 minutes), you will not have excess foods in the water. It does take quite a long time for a floating food to sink, and with a typically hungry koi that gobbles everything in it's path--this should not happen. I've seen horrible situations happen with snails taking over a pond because they are rampant breeders, AND even worse if any of them die it is often hard to tell for some time and you've got some rotting critter in your water. I just don't see the value. For an algae issue get a UV light.