Nitrite binds to the haemoglobin in the fish's blood and blocks the oxygen receptors. It does the same to fish as carbon monoxide does to us. There is no non-toxic form of nitrite as there is with ammonia. You need to remove the nitrite by water changes, I'm afraid.
You could try using Prime as your dechlorinator. This claims to detoxify nitrite though as they won't say what's in it, no-one can be sure. But you will still need to do water changes as the effect only lasts about 24 hours.
Get some Tetra Safe Start as this does contain the correct species of nitrite eaters. Pour it into the filter rather than just into the water.
There is another thing you could try but it's a bit fiddly. Salt, ordinary salt, can be used as the chloride part blocks nitrite from entering through the gills. It is best to use pure sodium chloride - that can be found in fish shops as 'aquarium salt'. The salt we use in cooking has anti-caking agents added, and in some countries iodate is also added.
Now for the fiddly bit.
- measure the nitrite level
- multiply it by 10 (this gives the amount of chloride needed for the nitrite level in mg/l)
- calculate the volume of water in the tank in litres
- multiply the numbers from 2 and 3 (this tells you how much chloride the tank needs)
- multiply 4 by 1.5 (salt is only 2/3 chloride)
The number from 5 is the amount of salt you need in mg. Divide by 1000 to get it in grammes.
Weigh the salt and dissolve in a bit of tank water in a tub. When it has all dissolved, pour it into the tank.
If the nitrite level goes up again, subtract yesterday's reading from today's to see how much extra nitrite is in the water, then use that extra nitrite figure in the calculation and add more salt.
When nitrite drops to zero, daily water changes will gradually remove the salt.