I originally had 7 ember tetras. I have a 40 gallon with platies, ember tetras, and threadfin rainbow fish. It’s a planted tank and the water parameters are fine. I did my usual 25 percent water change, and a day later the one ember tetra was acting strange and went into hiding, and I’ve never seen it since. All of the other fish are acting normal, greeting me at the front of the tank. I’m not sure what could have went wrong, could it just have been that fish?