Catfish can die and sit in the tank for a day or two before you realise they aren't alive. If you did a water change on Sunday, the pleco might have died from something in the new water or it could have been something else and was simply a coincidence.
The angelfish doesn't show any signs of disease.
I would monitor them over the next week and do some water changes. Maybe add some activated carbon to the filter for a couple of weeks and increase aeration/ surface turbulence.
Make sure nobody is using anything that produces fumes or gases in the room. Check for air fresheners, deodorants, perfumes, hair sprays, paint, cigarettes or anything else that releases fumes or gases into the air.
Make sure you don't have anything on your hands or arms when working in the tank. This includes things like perfumes from soap, moisturising creams, oil, grease, spices from cooking food, antibacterial hand washes/ soaps, etc.