Sorry that would be .3%.
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Salt is a superior remedy for many different reasons, many of which will show up in a search of this website and in the book I extoll this remedy even more.
Kosher salt, Sea Salt, Non-Iodized Table, Rock and Ice Cream Salts are acceptable. Do not use salt intended for livestock if it contains any minerals or trace elements.
Caution! No YPS or Yellow Prussiate of Soda should be in the salt. Prussic Acid is hazardous to fish because it dissociates into Prussic Acid in water!
Add the salt all at once if fish are dying off rapidly
Add one pound per hundred gallons, per day, for three days if fish are merely sick. This avoids occasional filter-shock. (2-3 day nitrite spike)
Will not harm hardy Lilies, common Papyrus, or Irises. Will cause yellowing of Hyacinths, Celery and Water Lettuce, but will kill Anachris, String Algae, Cabomba and Elodea.
Strengths
3 pounds per hundred gallons = 0.3%
6 pounds per hundred gallons = 0.6%
Pull sensitive plants to baby-pool and treat with Formalin in this. (25 ppm 8-12 hours)
Leave salt in the pond for 14 days minimum, then remove by partial water changes over subsequent weeks.
Note From Doc Johnson
"I am always absolutely sure to read every ingredient in the salt I am using. I do NOT take it for granted tha my time honored favorite brand of salt is still being made with the SAME anti-caking agent as always. They *do* change these ingredients occasionally and the addition of Yellow Prussiate of Soda (YPS) can cause death fairly rapidly in soft water!" ~ Doc Johnson