The amount of salt that one needs to add is actually very little compared to other applications you will see on the net. It is important to understand the relative risks of a little salt or nitrite poisoning. Part of deciding what to do has to with the bio-chemistry of nitrite poisoning. Basically, nitrite causes blood to lose its ability to carry oxygen. So even with sufficient oxygen in the water, it cannot get into the fish. What the chloride does is to block the nitrite for doing its thing.
Next, it helps to understand how nitrite leaves a fish. This will happen naturally over a day or two as long as there is no nitrite still in the water. So, lets look at two alternatives, a small amount of salt for a short period of time or 50% water changes. Lets assume one tests 1.00 ppm of nitrite in their water. Using the salt method, the ability of nitrite to enter the fish stops almost immediately. All that remains is for the nitrite already in the fish to work its way out. So the fish will improve fairly fast.
Now lets do a 50% wc instead. When done you still have .50 ppm of nitrite in the water and it is still entering the fish. So the effects will persist, even if reduced, for a longer amount of time. So you do another 50% change. And if nitrite is not being produced any longer, you further reduce the level to .25 ppm, which is still entering the fish.
This does not touch on the fact that one should only have detectable nitrite in a tank relating to cycling issues. So nitrite production may take more time for the appropriate bacteria to increase to where all nitrite is immediately converted to nitrate. Nitrite levels may rise before they fall. However, if one is adding salt and nitrite rises, all one needs to do is to add a little more salt. As soon as nitrite levels drop to zero, normal weekly water changes will remove some of the sodium part of the salt and wakly changes will be less invasive than every day changes or even more than one a day.
There is one last consideration in all of this. How much stress is the nitrite causing the fish? Equally important is when multiple big water changes are the method chosen to deal with nitrite, how stressful are these to fish which may be in a strange new tank environment and the water levels keep changing and maybe the temps as well. And what about the giant monster out there threatening them? (Hint: its the fish keeper doing water changes.)
Using your number of 33 gal tank and .50 ppm nitrite lets do the numbers using the directions I posted earlier.
1 . Multiply your nitrite test reading by 10. This will give you the needed mg/l of chloride you need to add.
.50 ppm x 10 = 5.
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
33 x .85 = 28.05 x 3.875 = 108.69 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.
5 x 108.69 = 543.45 milligrams.
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.
543.45/1,000 = .54345 grams of chloride are needed.
.54345 x 1.5 = .815175 grams of salt are needed. Call it .815 grams.
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.
"¼ teaspoon of salt shaker sized table salt weighs 2 grams." So, what you need is .815/2 =.4075 of 1/4 teaspoon or just short of 1/8 of a teaspoon of salt into what is essentially 28 gals of water. (The actual amount is basically .1 teaspoon of salt)
Most measuring spoon sets only go down to 1/4 teaspoon, some do have the 1/8 spoon. If all you have is a 1/4 fill it up level and then pour it into a pile. Do your best to divide the pile in half visually and use 1 half pile.
The tank size is 33 gallons (125ltr) and has been setted up for few month now with my new filter but hasn't been cycled before.