For “the shimmies” in guppies, you should almost always use Aquarium Salt (Sodium Chloride), not Epsom salt.
While both are “salts,” they perform completely different functions in an aquarium. Using the wrong one can actually make the situation worse depending on your water chemistry.
Why Aquarium Salt is the Right Choice
The shimmies are typically caused by Osmotic Shock—this happens when the water is too soft (low mineral content) or the pH has dropped. Guppies are hard-water fish that need specific electrolytes to maintain their internal balance.
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Aquarium Salt (): Adds essential sodium and chloride ions. This helps the fish’s gills function and reduces the energy they spend on “osmoregulation” (balancing the water in their bodies). This extra energy allows them to recover from the stress causing the shimmying.
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Epsom Salt (): This is primarily a muscle relaxant and laxative. It is excellent for bloating, constipation, or dropsy (swelling), but it won’t fix the electrolyte imbalance or pH issues that cause shimmies.