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If you have a planted tank, you want to keep the salt to a minimum. Just for routine use, I'd recommend no more than a teaspoon for every 5 gallons of your water change water.
Treating parasites like "Ich", then you need to double the amount to two teaspoons per 5 gallons and do a good job of vacuuming the substrate. Avoid vacuuming the areas around your plants. You don't want to damage the roots.
There are other considerations when dealing with parasites in the tank. But if you're just using the salt to benefit your fish and not treating parasites, then that's a topic for another time.
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