Aquarium salt is non-iodized sodium chloride (NaCl), used by freshwater hobbyists primarily as a medication for fish. It treats external parasites and promotes injury healing by supporting the fish’s natural slime coat. While beneficial for fish health, this salt can be highly detrimental to the physiology of live aquatic plants. Aquarium salt generally kills plants, especially at therapeutic concentrations.
The Mechanism of Salt Toxicity in Plants
The primary damage caused by elevated salt levels begins with osmotic stress. The high concentration of dissolved ions outside the plant cells draws water out, similar to physiological drought. This reduces the water potential around the plant’s roots and leaves, hindering its ability to absorb necessary water. The loss of internal water causes plant cells to shrink and lose turgor pressure, which is essential for maintaining structural rigidity and growth.
Following initial dehydration, secondary damage is caused by ion toxicity from the plant absorbing excessive sodium (\(\text{Na}^+\)) and chloride (\(\text{Cl}^-\)) ions. Sodium is problematic because it competes directly with potassium (\(\text{K}^+\)) for uptake sites. Since potassium is a fundamental nutrient, this blockage leads to a severe deficiency, disrupting metabolism and growth. The accumulating sodium and chloride ions also interfere with the plant’s ability to utilize other minerals like calcium and magnesium.
Plant Sensitivity and Observable Symptoms
Most freshwater species show a strong negative reaction to increased salinity, though sensitivity varies. Delicate, fine-leaved stem plants and mosses, such as Java moss, are the most vulnerable due to their high surface-area-to-volume ratio. Plants that primarily absorb nutrients through their leaves, like certain Cryptocoryne species and floating plants such as Frogbit, are also highly sensitive.
Observable signs of salt toxicity often mimic severe drought damage and can appear within days. Common symptoms include the browning of leaf edges (necrosis) and a sudden melting of the entire leaf structure. Plants may also exhibit stunted growth or general wilting as cells lose turgor pressure. For some plants, like Hygrophila difformis or Vallisneria spiralis, entire leaves may detach as the cellular structure collapses.
Practical Dosing Guidelines for Planted Tanks
When using salt to treat fish diseases, minimizing harm to plants requires precise dosing and a strict removal protocol. A low therapeutic dose (one level tablespoon of non-iodized salt per five gallons) is used for general stress reduction and is tolerated by hardier plants for short durations. Higher concentrations (one tablespoon per two to three gallons) are necessary to treat external parasites, and these concentrations will cause significant damage to most live plants.
Treatment should not exceed 4 to 7 days to limit the accumulation of toxic ions within plant tissues. Salt does not evaporate or get filtered out of the water column. To lower the concentration and mitigate plant damage, the salt must be physically removed through daily partial water changes. When performing a water change, only replace the salt for the volume of new water added to maintain the therapeutic concentration.