Female Endler's dropping like flies :(

[cybergoldfish]

A week ago, I had 3 male Endler's, 6 female and 2 babies in my 38 litre tank. Today, I'm down to 3 females. The first two deaths seemed unprovoked, there was nothing as far as I could tell wrong with them. The third looked like she was giving birth (she was pregnant).
I do a 40% water change once a week, and I've been treating the tank with myxazin since the first one died to see if that would work. The tank has:
NO3 = 10ppm
NO2 = 0ppm
pH = 7.6
I don't understand how to read total hardness and carbonate hardness, but they seem to be:
total hardness = >14º
carbonate hardness = 10º
(But I have no idea what these mean. Can anybody enlighten me?)
I've lost the measuring part of my ammonia kit, so I don't know that, but I regularly clean out, gravel vac thoroughly, wash filter material.

Any ideas what is killing my fish?

Thanks
James

 

[ADelphi]

I think we have something in common, did you buy from wholesale tropicals, i've already had one male endler die and another looks like its on its way out.

 

[cybergoldfish]

Replied to your PM

Yes, 2 of the 3 that have died were from W.T., and they've only started dying since adding the new ones.