Epsom Salt is dry and should be measured in grams, not ML.
It is something like 9.8% Magnesium
1 gram is 1000mg and 9.8% of 1000mg is 98mg
PPM is mg/L
1 US gallon is about 3.78L
So, 15 grams of Epsom Salt per Gallon would look like
15g x 1000mg x 0.098 / 3.78L = 388.89 PPM of Magnesium
This is an absolutely insane amount of Magnesium.
Most plants require a nutrient solution with between 20ppm and 70ppm. A strong Epsom Salt dosage would be 2.7g per gallon, or about 1/2 Teaspoon per gallon. I usually use 1/8th a teaspoon per gallon for the 12-15ppm boost I get from that. Even that small amount can be a 50%+ increase to the element over my base nutrient solution.
I hope you didnt feed that shit to your plants
The correct amount is 0g 0ml per gal until your plants tell you they need more than they are getting.
Peace
I throw a mcdonalds cheese burger in the bucket once a week. Those things are full of sodium. For my sulfur uptake, yup the eggs from an egg mcmuffin. High natural sulphur content. Put a grown man to sleep every time
Convert, 5 grams equals one teaspoon approximately. Like Snow says that's a shitload of mag,it seems less is more in coco.. I been using around a quarter teaspoon per four litres.. greens em up nice plus sulphur which is good during flower..Bud.
I've mixed my own salts for 15 yrs , and use .4 to .7 grams per liter as the only source of Mg
Your using too much , the ratio of exchangeable cations is important...