High Nitrite

Question
I have a 50 litre tank (2 ft x 1 ft).  It has been up and running for 6 months.  Up until recently I have had no problems with the ammonia or nitrite levels other than when the tank was cycling.  Suddenly for no apparent reason the nitrite levels have gone up.  To combat this I have done 20% water changes every day, added aquarium salt to help the fish, cleaned the tank fully including gravel, filter and ornaments, and have cut down the feeding to every other day.  I always use a water conditioner and leave the water to stand 24 hours before adding it to the aquarium.  At each water change I have been adding the beneficial bacteria to help the filter.  The fish do not seem to be unduly bothered by the high levels of nitrite but I am.  The ammonia levels is fine at nearly zero, and my water PH is 7.5. The levels of nitrite have been high for over a week now and I cannot get them down no matter what I do.  Help.

Answer
Carole,

 It could be a couple of thing. It could be your filter. Do you replace the filter media? Do you change the charcoal if you have any or anything else in the filter that needs changing? My first thought is your tank is trying to set up. Even though the tank has been running for six months it is trying to cycle again. When you did the whole tank cleanup including the gravel, you removed the bacteria that the tank needs. When you do that, your tank thinks its new and starts to recycle. When you do a gravel clean, do not go all the way to the bottom of the tank. The last inch holds all the beneficial bacteria and when you clean that, it gets removed. You do not need to add any bacteria in the tank, that is what the bottom of the gravel does, so I would stop adding it. I do not think the feeding has anything to do with the nitrites going up so its ok to feed your fish daily. Continue to do the water changes and I think you will be ok in a week or two. Watch the fish for any signs of a problem and if they do, just remove them from the tank and place them in your hospital tank. Not sure what kind of fish you have, but depending on if they need brackish water, there is no need to add the salt. If after a week to 10 days, the nitrites are still up, let me know.