First on the "salt" confusion; Barb is quite correct, Epsom Salt is not "salt" as in sodium (table or marine) salt.
"Salt" is a chemistry term referring to an ionic compound; you can read a lot more here:
Sodium salt, which is table salt and marine salt, is the "bad" salt. Other salts vary, depending what they are. Water is hard or soft, depending upon the various mineral salts it contains, such as calcium salt and magnesium salt making it harder the more there are of these two salts. Read more in my article here:
Second, on Epsom Salt. Yes, this is good for plants--provided it is pure Epsom Salt [magnesium sulfate] and contains no additives, and in moderation. One does have to be careful for the reason Pew mentioned, and I have had personal experience with this as I'll explain.
Epsom Salt is magnesium sulfate, or the salts of magnesium [that second mineral that hardens water] and sulfur. Plants need both of these. Back in the 1980's I read in an aquascaping column in FAMA magazine that adding ES to a planted tank would give better plant growth, and in those days that was all the fertilizer I used. And the plants were fine. I had very soft water, so the slight mineral hardness was insignificant, though at the time I had no comprehension of hardness.
I cannot remember the amount I used, but it was no where near the 1 teaspoon per gallon mentioned previously in this thread. I would not add that much, you will have serious fish trouble due to the sudden increase in hardness. At on this, I have found that in my present water which has < 1 d GH, using one teaspoon for every 30 gallons raises the hardness by 1 dGH. So in a 30g tank, adding 3 teaspoons will result immediately in hardness of 4 dGH in water that now has 1 dGH. And in a 90g tank adding 3 teaspoons will raise the hardness from 1 dGH to 2 dGH. Etc. However, this is in my 1 dGH soft water; I have never had "hard" tap water so I have no idea what effect this might have in already-hard water. Given that magnesium is one mineral that creates hard water, it is probably sufficient in most tap water as it is. Sulfur is often present in tap water too. Both of these are trace elements or micro-nutrients, meaning that they are not required in large quantity by the plants. [Never quit, you mention 80ppm which is around 4 dGH. That is perfect for soft water fish, so I would not want to mess with that.]
Which brings me to the fertilizer issue. Plants need 17 nutrients in proportion to each other. An excess of some can cause plants to shut down assimilation of others. And i had this occur in one tank. By adding ES only as 2 teaspoons in a 90g tank, I created an overdose of magnesium and the swords displayed an iron deficiency, since excess of magnesium can cause this because the plant shuts down on the iron assimilation. So my advice is, if you have very soft water, using ES will help since it will also raise the GH slightly, and might be sufficient on its own for the plants. But otherwise, I would go with a complete fertilizer like Seachem's Flourish Comprehensive Supplement.
To avoid mixing issues, I'll respond separately on the light question.