Backyard Aquaponics
A place to discuss aquaponics

How does epsom salt help plants and fish?

Not sure about the fish but epsom salts provides magnesium which is used by the plants. The section on "How do plants use Magnesium" located here will answer the plant portion of your question.- https://www.agrocares.com/en/news/magnesium/

Probably wouldn't be too difficult to look up how it's used by fish but I'll leave that to your or someone else.

Cheers

I've added a product called Grow Big, from Fox Farm Co. I was told people into Hydroponics use this all the time. Sense I added this I've been able to harvest cucumbers, bell peppers, Cheyanne peppers, and jalapeno peppers.
Provider beans and peas quit producing, so I'm still looking for something else to do.