Rock salt, or aquarium salt, or sea salt are used by several aquarist to reduce attacks from parasites like ich, costia, anchor worms, skin flukes; or for the purpose of osmo-regulation which in turn helps an injured fish to recover from scale loss or injuries; or for enhancement of slime coat of the fishes as cure/prevention of ailments. I am not well versed enough in fish pathology to make comments about the efficacy of these treatments and I shall not comment about it. I am only concerned as a gardener of aquarium plants how such salt affect the plants.
Freshwater, as in rivers and lakes, does not have appreciable salt content in nature. Most of the plants that we see in an aquarium come from such habitats. As these plants have evolved in freshwater, appreciable amount of salt and/or salty water must be a foreign condition to which a plant has to adapt if it can. Let us examine what salt would cause.
We know that higher amount of soluble solid content increases the internal pressure in the water, demonstration of osmosis in physics have shown how water with lesser amount of solids in solution tends to equalize by escaping through a semi-permeable membrane into a more concentrated solution on the other side. The outer skin of an aquatic plant is semi-permeable. So when we add salt to the freshwater aquarium in any appreciable quantity, there is a chance that a plant will loose its internal water to its surrounding. In other words the plant will wilt.
The freshwater plant cells lose its water to the salty water by osmosis, and the cell membrane of the plant pulls away from the cell wall, a phenomenon called ‘plasmolysis’. This happens because the cell membrane is selectively semi-permeable and does not allow salt to pass into the cell but allows the water to pass through it freely. If this state of affair remains in this way for any appreciable period of time the plant shall die.
I have come across the comments of several aquarists saying that they just cannot grow even the easiest of aquarium plant. When I read such comments it comes into my mind that ‘1 teaspoon per 10 gallon’ salt addition advice. Salt does not evaporate from the aquarium once it is added. More salt with water changes and topping up, if unregulated will simply increase the salt content to a level where it shall make it hard for freshwater plants to survive.
So next time those of you who find it hard to keep plants in your aquariums, try your hand at planted freshwater aquarium, go lightly with salt, and see how easy it is to grow plants. A sand and gravel substrate to anchor the plant, and a little light about 1.5 to 2 watts per gallon and you will enjoy the beauty of a planted aquarium and your fish too will thank you for it.