No clue to whats wrong

[Icysword]

Thank size : 48x18x12 inches
Filtration : Eheim 2217 and 2215
Temperature is at 27 deg
Tank has been setup for around 8-9 months, with the scape changing recently in the past two weeks.
Setup is with rocks and driftwood.

Lost 20 Rummynose, 3 L series, 8 Ramarezi, 3 Apistogramma.
Remaining few fishes are just sitting in a corner, with stress bars and faded colors. Not eating or moving around.
Loss happened within one day.
The only thing that changed was a normal water change.

Running Carbon as of now extra in the tank.

Any ideas on how to proceed further?

 

[xraycer]

Sounds like poisoning. May be some sort of contamination in your water. I think all you can really do is keep doing daily water changes and hope the contamination is gone.

Have you check the pH?