Dead Corys and Barbs

We recently bought a 10-gallon tank and let it sit empty for two weeks. After we got all the chemical levels normalized, we put in two corys, 4 green barbs, and a plechto. Within a few days the nitrite levels went way up and despite putting in the nitrite reducer the barbs died. Since we were hesitant to introduce any new fish until we knew what was happening, we left the others in and continued to monitor the water. All was going well for about a month when both corys developed sores on their undersides and died within 12 hours. The plechto is thriving. Any ideas what we are doing wrong here or what caused this? In case it isnt obvious we are new aquarium owners so be gentle

 

what are your reading's ?
Are you doing water changes ?
Your tank is in the process of cycling this will take a while

 

reply: Dead Corys and Barbs

All of our readings are good now, but previously the nitrite level was high. We have been doing cleanings and 40% water changes. About how long will the cycling take?

 

a normal cycle should take 25 to 45 days
i wouldn't do that much of a water change do 15 to 25 percent
i would not clean your filter media during this time if you do clean it used water from your tank