the best way to use epsom salt is
get ya epsom salt and put in a cup fill up with boiling water and stir untill completly disolved then add it to ya bottle of cold water this will get rid of the heat from the boiling water
then it is safe to use
i have found that spraying it on the leafs when the lights are off helps a lot aswell as pouring on the soil
and have seen the results as soon as 2 hours after
hope this helps
cheers all