Nitrite in tank dangerous high. wont go down.?

I have a ten gallon fish tank and when i did the fish test it said the nitrite was stressed. and a fish died. the next day another fish died. So that day i went and got a stress chemical and its been about 3 days and the stress has gotten higher and another fish died this morning. I have already used the gravel cleaner and changed out the water 25% and have changed the filter. Please tell me what to do