Yes 50% now then another and another. Wait half hour test again and if nitrite are over .25ppm do more. Your fish are in a very toxic environment. It would explain what happened to your fish last week.
Letting the tank sit with water and filter running for a week is NOT cycling. You need an ammonia source. You are now fish in cycling. Check all your other tank too.
http://www.aquariumadvice.com/articles/articles/124/2/-I-just-learned-about-cycling-but-I-already-have-fish-What-now/Page2.html
You need to keep nitrates under 20ppm.
Edit: MAKE SURE TO USE TEMPERATURE MATCHED, DECHLORINATED WATER.