Bigger issue...
When was your tank setup?
What is your setup (Tank size, etc)?
How long ago did you start placing livestock into the tank?
It sounds like the fish are not a mated pair, just a pair, which means very little. However, the fact that they were in the same tank at the store is a good sign.
Tanks go through a natural cycle. I am not going to explain this, as it would be easier for you to google it or read about cycling on here....if that is the case, you will need to get everything that is living out of your tank....it is also possible the rock you placed into the tank is curing or caused a mini cycle.