All loaches dying within a day from each other
I have owned three yoyo loaches and a zebra one for four years with no problems of any kind. they have been living alongside three other races for nearly six months now. however, yesterday we noticed one of them was dying, then this night another one died too, and we have our third yoyo dying as well right now, which leaves the zebra well and alive. they all died within a day from each other and hours before their deaths they didn't even seem to be sick at all. what they did was lie on their side mouth open and breath extremely fast. getting them near the surface changed little. what I tried to do was poking them to see their reaction, which was none, separating them from the community tank and, when picking them up, they did not even react to being pulled out of the water. The point is, this is just happening now, my dad has been keeping loaches within other fish for nearly 50 years and this was news to him too. another thing I have noticed is the healthy ones seemed to be lying next to the currently dying ones until death.
my father changed half of the water amongst other things and followed his usual procedure, my take on this originally was that our plant growth is taking up all of the oxygen, then again this would affect the other fish as well, which is not currently happening. Temperature is not the problem, either.
at this point we are just waiting for the zebra to eventually die, my father is also worried that the other fish we're keeping could too. I did not find a thing about this loach behavior on the internet, which always stated they were just sleeping when mine have indeed all died. pic for reference