This is the simple explanation of why.
I would like to point out that epsom salt is just called a salt because of its grain rather than any actual properties in it. There's no sodium in it but rather its just MgSO4 which is Magnesium Sulfate; it's perfectly safe for salt intolerant critters.
Epsom salt is the market name for hydrated MgSO4 and adding 1 milliliter of it per 10 liters of water will increase the permanent hardness by roughly 70 mg/L CaCO3.
Salt should only be added to treat certain conditions, stress reduction associated with physical damage and/or high nitrate, hydra control, ulcer disease and aids in the removal of leeches.
I tried to discourage the use of salt because it ticks none of the above boxes.
I can't see how the fish will benefit? Salt of any kind will affect water balance, predominately (95%) water is made from 8 ions, - charge C, SO4, CO3, HCO3 & + charge Ca, Mg, Na, K.
There are 70+ trace elements to make up the rest of the "water" (5%)
The relative concentration of these ions determines hardness and salinity, hardness has a direct link with pH.
Addition of any of these major ions will affect the chemical balance of the water without any question of doubt. From the evidence provided earlier in the thread it would appear this fish is not producing enough correctly coloured faecal matter, this is not right for this family of fish, when they "go" it is clearly evident and abundant.
Ask surfer who has myriad plecs, I have kept five different species from 4 subfamilies.
This fish is potentially wild caught or has been kept with wild caught specimens. Most if not all wild fish carry bacteria and parasites that among other things can also effect buoyancy and the digestive tract. As the family Loricariidae has a really long digestive tract to deal with all that vegetable matter, bacteria here can release a gas, leading to floatation issues. That is my opinion, backed up by a great deal of research, not only on this family of fish but also concerning water chemistry and parasitic and bacterial infection in fish and the treatment of such ailments.
Edit- Mg is magnesium, an alkaline earth metal, So4 is sulfate, sulfate is a "salt" of sulphuric acid.
Combined you get MgSo4. Magnesium sulphate.