[sftbl3]

I have a 60g Mbuna tank. The tank has been up and running, stocked for almost 9 months. After it was cycled I got 4 yellow labs (3:1), 4 Rusty's (3:1) and 4 Saulosi (3:1). Immediately I had ick. I treated the tank as is customary but in the end lost everything except my yellow labs and one female Saulosi (Nancy Saulosi; see what I did there?). Well this morning I found her dead. Looked her over and couldn't see anything obvious with her corpse. Water quality is where it always is. Just did a 40% H2O change 4 days ago. I added a sponge filter to the tank so I could eventually use it for my QT because I wanted to add more fish to this tank. I don't think it's disease because I have 8 fry of varying age and size in that tank and they are fine, swimming around like they own the place. Could her death be due to the fact that she was the only one of her kind in the tank? I never witnessed any aggression from the labs. Just curious what your thoughts are and looking for advice if anyone has some.
Thanks in advance.
:-?

 

[sftbl3]

pH 8.2
Ammonia 0
Nitrite 0
Nitrate 40 ppm (not sure why this is so high)
Water temp. 81f
Tank dimensions: 48x25x12.5

The labs are definitely 3 females and 1 male. The Saulosi was definitely female. When I originally stocked the tank I got "ick" immediately and lost all of my rusty cichlids and 2 female and the male Saulosi. My tank has been great...all 3 female labs have had fry. I've got a total of 9 fry in there now and they are fine which is why I assumed that the tank water was ok or they would have been taken out first. She was acting strange on water change day, she was hiding in between 2 rocks which was unusual for her but I just figured she had had enough of me that day because I also added a sponge filter to the tank so I could prep for starting a QT tank and add fish to this tank eventually.