15 fish died after one week away

So, we just got home from a weeks vacation and most our fish have died. They all look like they're gasping for air, and the few survivors have gathered at the top. A few days before we left we changed the gravel to AquaSafe (I've attached a photo). There has been a power outage, only for a few hours, but the pumps are still going. Temperature is normal. The heater is still functioning. They haven't gotten any food during the week, but we've left them alone for longer without any issues many times before. The fish are Malawian cichlids. We have about 15 dead fish and 6 alive.

Following numbers were measured but can be affected by the dead fish rotting: 0,8: Cl2 mg/L 7,6: pH 20: KH GH: <21° NO2: 10 mg/L NO3: 100 mg/L

We would really appreciate any help anyone could have. We've had this aquarium for many years now and it's really sad to see.