One of the subjects that always generates a great deal of debate, and where all aquarium hobbyists have an opinion, has to do with adding salt to freshwater aquariums. Many “experts” on the Internet opine that salt should be added to all freshwater fish tanks. Many fish wholesalers add salt to their tanks. What I will give you here is just my opinion, from many years of experience handling aquarium fish.
1. As a Tonic for Hobbyists
This is an easy sale for the store, with much profit, but also much danger. Hobbyists don’t always understand why they do something and, often at the recommendation of the LFS (local fish store) or someone on an online forum, they will add salt as a general tonic. They often do not understand that as water evaporates the salt remains, and when they add water they keep adding salt. Eventually, they have a brackish water tank and fish start croaking.
2. As a Cure for Ich
Some aquarium hobbyists are convinced that the best cure for ich is to raise the temperature into the 80 degrees Fahrenheit, and add lots of salt to the fish tank. My take is that when fish are already stressed with a protozoan, why would you want to stress them more by raising the temperature and adding salt? There are plenty of excellent remedies for ich – and salt with high temperatures is not one of them.
In the in-store fish tanks used for holding and selling fish, salt can definitely be useful. Most livebearers, especially guppies and mollies, have been raised in a fairly salty environment, especially fish that come from the Far East. Guppies and sailfin mollies will simply not thrive unless they have salt in their water. Fancy platies and swordtails that come from the Far East also really need salt.
What to use for salt? Here I can tell you what has worked well for me – I learned this from the guy I transship my Far East fish through. Morton’s offers a salt called “System Saver II Pellets” that is made for use in water-softening systems. It comes in 40-pound bags at home improvement stores. I use it at the rate of two pellets for five gallons of water, and it has completely solved any problems I used to have with fancy guppies and mollies.