Female plecos dying mysteriously

[Nik_n]

Hello

Over the last two days two of my bristelnose pleco females have died. It was very sudden and surprising, since they didn't show any sign of sickness and were actively breeding after new years. I had two spawns from the fish in less then a month but I noticed there was no breeding since then. I feed them algae waffers as well as zucchini. Both had some nipped fins but neither had any fungus or showed any difficulty swimming. 

The water perimeters

Temp:27°C, NO2:10-15ppm, NO3: 0, ph=7.6, gh: 4-7° kh:15°

The other fish seem fine, I will do a large water change tomorrow afternoon.     

[Guppysnail]

Are you certain they were nipped? Was there ammonia. If nipped what else is with them or have they shown any aggression towards one another?

[Nik_n]

It definitely looks as if they were nipped. I'm assuming the angles did that, but I didn't think much of it as it happened before but healed up really quickly. I didn't do an ammonia test (ran out need to get some new ones ASAP) but the male is completely fine and so are the other fish. Did a large water change today (2/3) and did a gravel vac as well.