It seems this has to do with whatever happened with the water change start by checking the water with the test kit, check to see if you forgot to do something after the water change like turning off the heater by mistake. Did you forget to add dechlorinator sometimes people forget to do this, I have once or twice in the past 15 years.You might want to check your equipment make sure nothing is broken.Is the mayan pushing them around maybe they are just threatened by him.