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Is epsom salt(magnesium sulfate) safe to use on corydoras? I need the magnesium to fertilize my plants. I know it's not safe to use salt on corys but not sure if this applies to epsom salt.

Epsom is not salt like you use for sw tanks or to medicate in fw or to season food. However, it will raise the TDS of the water. Low level dosing being taken up by plants should not be a problem.

But, if you are dosing Flourish or Tropica Plant Nutrition or a similar micro-nutrient product for plants, they contain magnesium.

If you look at products such as Seachem Equilibrium, it's ingredients list magnesium as one of it's components, and then "Derived from: potassium sulfate, calcium sulfate, magnesium sulfate, ferric sulfate, manganese sulfate."

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Mats

Note that plants use these minerals in "trace" amounts, not ever would you need to supply Epsom Salt(magnesium sulfate) or other minerals in high enough doses to affect Corydoras or other fish.
That Corydoras are particularly sensitive to common rock salt, Sodium chloride, is an old fish wives tale. They tolerate it as well as any other common SA aquarium fish. There is no reason to add salt unless it is for a particular effect and adjunct to the treatment of some injuries or diseases. It is very rarely essential, even then.

yeah I was just going to use a small amount, so I guess it's safe. Thanks