Your fish looks to be suffering from multiple bacterial issues...Most importantly you want to provide clean water for your remaining fish by doing 10% daily water changes until your bio-filter recovers; You could also add zeolite to chemically purify your water.
What and how much where you feeding them? Pebbles don't just accidentally end up in a fish's stomach. If you weren't feeding enough or feeding them the food with proper nutrients that can explain the poop color. And a 90% water change with a power outage and low oxygen mixed with being hungry can be something hard to overcome. That's assuming it's something with them eating. Even if you were feeding them the right stuff and everything were they accepting it?
I was feeding them a mix of omega flakes, hikari pellets, and algae wafers. I guess my luck ran out this month but at least I can start over with possibly new batch of african cichlids