[pleco_breeder]

Hello,

This is beyond what is normally taught because it's really a bad idea to let water get out of control. However, fish can, and will, adapt to water quality parameters as they get outside the expected ranges slowly. An abrupt change kills everything, but slowly degrading water forces current inhabitants to change. I would strongly recommend testing ammonia/nitrite/nitrate as this sounds like what has happened with your tank.

Larry Vires