I just got home from work and one of my rbp died....when I got here she was still swimming but all over the place facing up and down and rested on one of the rocks.....then she just died....2 other piranhas are fine but one is away from the group at the very top of the tank.....they all seem kinda slow.......I check the water and amonia is 0 nitirte 0 nitrate 10 and ph 7.6.....please help I don't want to lose anymore fish