First, I am so sorry for all your fish losses!
I find it odd that you have a NitrAte reading of 0, is that normal for you? Have you tried testing your tap water to see if that might be the problem?
Also, what is your pH, both aquarium and from the tap?
We are at about 7.1 or 7.2 on tap, aquariums read the same. My cichlids have always done well with this ph, even though they require a little higher. Nitrate reading of the aquariums read between 5 and 10 ppm right now. Sorry. I have been cleaning out the dead fish all day and changing the water. We have done 2 50% changes on the 55 because all the fish are out of that tank and in my 2 10 gall hospital tanks. The 100 gall has had 1 50% wc, but I have no hospital tank big enough for my tinfoil barbs. So they have stayed in there. They are doing good, no signs of illness, and they are eating like champs. Thanks for the reply.