I had read somewhere (can't remember where now) that salt is good to use when you have nitrites. I decided to do a little research on that and it turns out that (according to a couple white papers I read) if you have nitrites it is very important to use salt.
So, here is the justification: Nitrites absorb through the gills and turns hemoglobin in the blood to methemoglobin. Methemoglobin does not allow the blood cells to carry oxygen, so basically the fish suffocates.
Chloride in salt competes with nitrites and basically blocks the uptake through the gills of the nitrites. Therefore, the hemoglobin is protected and the cells can continue to transport oxygen. You apparently need about 9 times more chloride than nitrites to give your fish good protection.
I also read that 1 PPM of salt equals .80655 PPM of chloride, so 1.24 PPM of Salt = 1 PPM of chloride. So, if you have 2 PPM of nitrites you would need 18 PPM of chloride which would equal 22.32 PPM of salt or .002232% (22.32/1000000) salinity (not a whole lot).
So, lets say you have a 55 gallon tank with about 5 gallons or so displaced by stuff in your tank (like me
). That means you have 6,682 ounces of water (if I did my calculations right). In order to reach .002232% salinity you would need to solve a little equation: x / (6,682 + x) = .00002232. So, according to this information you would need to add about .15 ounces of aquarium salt to your tank in order to protect against 2PPM of nitrite in a 55 gallon tank with about 5 gallons of water displaced.
I, unfortunately, do not have a food scale or anything, so I was unable to see how much a tablespoon of aquarium salt weighs. But, on the API box it happens to give you the number of gallons it treats, the total weight of the salt in the box, and a guideline for how much to add. I figured they would calculate the number of gallons it treated based on the smallest amount it tells you to add (looks better to have a bigger number). So, using that information I calculated how many tablespoons total there were in the box, and divided the total ounces by that and came up with about .9 ounces of salt per tablespoon (if anyone has a scale and could measure a tablespoon of API aquarium salt for me that would be awesome!).
So, if you follow the API general recommendation for tropical fish you are adding .9 ounces for every 5 gallons of water. That comes out to .9 ounces of salt for every 668 ounces of water. This equals: .9 / (668 + .9) = .00135 or .135% salinity. This also equates to 1,350 PPM of salt or ~1088 PPM of chloride.
Anyway, my point is that there is a very good reason for using salt if you are having nitrite issues. Another point I'm trying to make is that it takes a very small amount of salt to protect against nitrite, and the API recommendations is far more than needed. So, if you have nitrite issues using the recommended API dose should be more than enough protection. However, based on the information I read using salt is not fool proof and some fish can still have problems and die due to nitrite. So, the use of salt is no replacement for good maintenance of your aquarium as the best protection against nitrites is just not having them. But, if you get in a pinch (like me) and need to use it, it should offer some good protection against nitrites.
I just want to note that my calculations depend on a tablespoon of aquarium salt equaling .9 ounces. That calculation was based on some assumptions that may prove wrong, so, like I mentioned before, it would be awesome if someone could actually measure it for me.
One more number it depends on is 1 PPM of salt equaling .80655 PPM of chloride. I can't remember where I got that number from, just had it written down in my calculations, so if anyone knows that is wrong let me know so I can recalculate.
Also, I forgot to save the papers to my favorites, and I'm going off all the stuff I wrote down yesterday. I'll try to dig those up again and post them. I hope this helps someone. I did a decent amount of research over the past couple days because my nitrites are starting to spike (for those of you who haven't read my other posts, I accidentally destroyed my cycle) and I wanted to offer my fish some protection. So, I figured I'd post some of the stuff I learned.
So, to answer your question. You should probably use your salt by keeping it around in case of emergency.
EDIT: Found one:
http://www.ca.uky.edu/wkrec/NitritePonds.pdf
!!!
