When I first read this thread I enlarged the pic of the Hygrometer and the reading was 1.004. It is so low that it should not harm the plecos that fast and certainly should not have killed them. However, it might exacerbate a more dire condition.
Marine aquariums are also measured in specific gravity. Specific gravity may be defined as the ratio of the density of the liquid to the density of water. Since density of liquid varies with temperature, so does specific gravity. It has been determined that liquids with a specific gravity less than 1 are lighter than water; those greater than 1 are heavier than water. The specific gravity of seawater at 35 ppt is 1.026.
The proper range for a marine aquarium is 1.020 to 1.026.
The proper range for a brackish aquarium is 1.005 to 1.015.
If salt were the issue I would expect the other fish in the tank (save the mollies) would not like it either, esspecially tetras and yoyos. I would expect if the level were much of an issue the other fish would show some sign of distress or not acting normally. When I first got into the hobby I added a couple of mollies to my tank. My water is close to neutral pH and soft. They did not do very well and I lost them inside a couple of weeks. I could not add salt as I had live plants and other fish that would not like salt. Some mollies may do OK in saltless water but others have been used in the past to cycle salt water tanks.
That said, I would think that there is no reason to add salt to most FW tanks save those with rift lake cichlids. Salt become more of an issue realtive to evaporation and topping off. Salt does not evaporate.
There is seldom a need to add plain 'aquarium' salt,
and there is normally less reason to measure it in a FW tank
I am also very curious as to what specifically is being added.
I would not expect it to cause issues long term with the fish, but potentially issues stemming from acclimation. I would maybe consider testing the water from the store for everything to see how that compared to the aquarium water.
or I would just give up, because I'm not a real fish keeper anyway
