Q. I just got through reading an article on sailfin mollies in which it said they do well in planted aquariums. It also said they need water with certain salinity and that you should run airstones for them. Forgive me if this is a dumb question, but isn’t it true that freshwater aquatic plants would not stay alive in salted water and that airstones would drive off the carbon dioxide they so desperately need?
A. Sailfin molly fish (Poecilia latipinna) are nice for planted aquariums because they tend to eat some forms of algae but do not bother the aquatic plants. They have a gentle disposition and get along well with a wide variety of other fish, provided they are not kept with fin nippers attracted to their long, beautiful fins. Sailfin mollies tolerate a wide range of water conditions, including water that has little oxygen. They get their share using water near the surface with their upturned mouths.
Their natural environment, however, is often marshes and other locales that tend toward brackish. They will breed in brackish water in the planted aquarium. Water that has up to about 17 parts salt per thousand parts water (ppt) is considered brackish. Freshwater would normally have less than 0.5 ppt of salt. Ocean water is about 35 ppt. Put another way, sea water is about 3.5-percent salt. One teaspoon of salt per gallon of distilled water equates to about 0.132 percent salinity, and a tablespoon per gallon is about 0.4 percent salinity.
Generally, aquatic plants do not tolerate much salinity. Too much salt can destroy plant cell structure, resulting in withering and death. They can withstand a few parts per million (perhaps as much as about 1 part sea water in 7 or 8 parts distilled water). However, it gets trickier if you go higher. Some freshwater aquatic plants, such as the Bolbitis species, can stand as high as a tablespoon per gallon, whereas others, such as the Hygrophila species, need much lower levels.
So the ideal, if you want to keep sailfin mollies in a planted aquarium, would be to have only a slight amount of salt, maybe a teaspoon per gallon. This would be a bit of a compromise between plant and freshwater fish preferences. If any of your aquatic plants react badly, change plants, or add some freshwater to reduce the salinity.
Cell damage isn’t the only problem that salinity can pose for freshwater aquatic plants. If you add sodium chloride (NaCl) or sodium bicarbonate (NaHCO3) to very soft water (a GH of less than 4), the sodium can interfere with the aquatic plants’ uptake of calcium. The result would be deformed growth in some hard water plants, such as vals (Vallisneria). So, if you add a small bit of salt to your planted aquarium and you notice deformed growth, try adding some calcium to balance the sodium for the aquatic plants.
Regarding carbon dioxide, if you are adding it to your planted aquarium with a fermentation bottle or a compressed gas setup, then you are trying to aim for about 20 to 30 ppm CO2, or about five to 10 times what would be in the water without added carbon dioxide. Surface turbulence will cause the water to shed carbon dioxide more quickly than otherwise, making higher levels more difficult to maintain. Airstone bubbling is a form of air and water turbulence that has the same effect. On the other hand, if you are not adding carbon dioxide but relying on fish respiration, and more importantly, the atmosphere as a source of carbon dioxide, then aeration is not likely to deplete the relatively low carbon dioxide levels under such conditions.