Salt can treat a number of disease organisms that affect fish and shrimp and is generally very safe for them.
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The OP said they had a bunch of snails in the tank. If that is the case, the snails will be adding to the bio-load in the aquarium. Snails eat and poop in the tank and use oxygen and produce ammonia just like fish and shrimp do. Having a couple of snails won't make much difference but if there are 20, 30, 50, 100 snails in the tank, it will add to the crap being produced.
One average sized Mystery snail produces as much bio-load as 3 or 4 neon tetras.
A couple of adult Malaysian trumpet snails equal 1 neon tetra.
One adult Ramshorn snail equals 1 neon tetra.
Snails can quickly build up in numbers and I have seen tanks with thousands of snails in and they used to cause the water quality to go off really quickly. And the amount of poop they produced was huge. And before everyone says it's food related, these tanks were full of plants and didn't have many fish, and didn't get much food added to them. The snails ate the algae and plants and eventually destroyed all the plants.
People under-estimate how many snails are in their tanks and the amount of waste they produce. And if there are no predators to control them, they will breed out of control.