Just a heads up. My ompression when I read this thread the first time was nitrite poisoning. If you get an ammonia spike, you will get a nitrite spike as well. The way nitrite works is it gets into the bloodstream of the fish. Once there it turns hemaglobin to methaglobin and it cannot hold oxygen. Fish suffocate because their bllod wont carry oxygen even though there is plenty in the water. Once insude the fish nitrite takes a day to work its way out. However, if there is still nitrite in the water, it will continue to enter the fish. Chloride in the water blocks the nitrite. Salt is sodium chloride.
The best way, imo, to soften water and to help lower pH is by using some amount of ro or ro/di water. I uses this for my Altum angel tank.
I also authored severa cycling related articles in 2013 for another site, They are still there but I left the site soon after. Part of those articles was a step by step set of instructions for how to use common salt to deal with nitrite. Here is is:
Fortunately, there is an effective way to blunt the harmful effects of elevated nitrite that doesn’t involve changing lots of water- you add salt (sodium chloride) to the water. The chloride in the salt acts to” block” the ability of nitrite to enter though the gills of the fish and thus to cause the harm inside the fish it might. So it is possible to manage elevated nitrite over the short term using salt in relatively small amounts.
“Sodium chloride (common salt, NaCl) is used to “treat” brown blood disease. Calcium chloride can also be used but is typically more expensive. The chloride portion of salt competes with nitrite for absorption through the gills. Maintaining at least a 10 to 1 ratio of chloride to nitrite in a pond effectively prevents nitrite from entering catfish.”
It should be noted that the Merck Veterinary manual suggests a lower ratio of chloride:
“In freshwater production ponds for channel catfish, a ratio of 6 parts Cl to 1 part NO2 has effectively prevented or treated methemoglobinemia caused by nitrite exposure.”
Since the amount of salt needed to produce either 6 or 10 times the chloride as nitrite is minimal, this author prefers to use the higher ratio of 10 to 1 in order to be more certain of obtaining the needed relief. One should also be aware that studies indicate that, whether one changes water or uses chloride to counter the effects of nitrite toxicity, it will still take between 24-72 hours for nitrite already inside fish to be completely eliminated. Preventing further nitrite from entering usually solves the problem.
WHAT SALT TO USE
Plain old table salt is just fine for use here. Do not worry if it says Iodized or if it says it contains Anti-caking agents. The amount of either of these in the salt is so minimal one would pickle their fish long before these ingredients would be doing any harm. Use small grain salt not rock salt. Kosher salt is good.
CALCULATING HOW MUCH SALT TO ADD
PPM is a measure of concentration in water. You cannot weigh ppms. However, 1 mg/l is almost the exact equivalent in water to 1 ppm. So one can use ppm and mg/l interchangeably in this case. You can weigh milligrams.
To add 10 mg/l of chloride for every ppm of nitrite in the water, use the following steps:
1. Multiply your nitrite test reading by 10. This will give you the needed mg/l of chloride you need to add.
2. Calculate the actual volume in litres of the water in your tank. If your volume is in gallons you must convert this into liters. (As a rule, using the advertised volume of the tank at about 85% will put you in the right ballpark.) 1 gallon = 3.875 litres
3. Multiply the number in #1 above by the number of liters of water in #2 above to get the total number of mg of chloride you will need to add.
4. Because salt is roughly 2/3 chloride, you must multiply the number calculated in #3 by 1.5. You now know how many mg of salt you should add to the water. Dividing this number by 1,000 will convert this amount to grams which are easier to weigh for most people.
5. Do not add the dry salt directly to the tank. Remove some tank water to a container and mix the salt in that, then add the salt water to the tank spreading it around the surface.
Hint: We have calculated a handy conversion from grams to volume so one can measure in tea or table spoons which most folks are likely to have while a gram scale is not. The following calculations were made using an Ohaus triple beam scale: ¼ teaspoon of salt shaker sized table salt weighs 2 grams.