I'm not sure I understand 33ppm salinity. Is that the sodium level of the water? 33ppm is probably about the net amount of salts used as fertilizer in an EI dosed tank over the course of a day or two.
In my past experience with saltwater, it was always measured by specific gravity. SG measuring floats are cheap, and readily available at most aquarium stores.
I know my EC meter reads around 200ppm from my tap water, and it's most definitely fresh water. It's primarily calcium and magnesium in our local water.
Epsom salt is magnesium and sulfur, and is often dosed at high levels to relieve constipation in fish. Dosing to 33ppm with epsom salt would keep animals from getting bound up, but would not be in any way similar to the mineral profile of normal brackish or sea water.
Maybe I don't understand the units of measurement that lead to a level of 33ppm salinity (haven't kept saltwater in years), but what you're about to do sounds a bit strange to me.