what kind of filters you use? I have a rena filstar 3 on my main planted tank and I change the pads once a year, and clean them half way through. I have a small HOB attached for water polishing/chemical filtration that I can change whenever I want.
the barb die off could have triggered a small cycle. if you have nitrites your tank is not completely cycled. do you have any ammonia? even trace amounts are toxic to fish.
Because a "cycled" aka mature tank has ammonia & nitrite readings of 'ZERO' at all times. Nitrates will vary depending on your water change schedule but the 1st 2 named are always '0' in a cycled tank.