There's no point chucking everything out unless you know what killed the fish. If the pleco is still alive, then it isn't poison.
Without seeing the sick/ dead fish they could have died from anything. Guppies are riddles with external bacteria and protozoan infections when they come into shops. they also have gill flukes and intestinal worms. Bettas are genetically weak from 50 years of inbreeding. The fish probably died from a protozoan infection that could have been treated with salt or another medication.
If you want to disinfect the tank and equipment, soak it in salt water for 24 hours. Then rinse with fresh water. Move the pleco into another tank when you do this.
Without seeing the sick/ dead fish they could have died from anything. Guppies are riddles with external bacteria and protozoan infections when they come into shops. they also have gill flukes and intestinal worms. Bettas are genetically weak from 50 years of inbreeding. The fish probably died from a protozoan infection that could have been treated with salt or another medication.
If you want to disinfect the tank and equipment, soak it in salt water for 24 hours. Then rinse with fresh water. Move the pleco into another tank when you do this.