He didn't make it into my reef which is right beside the octopus tank all those fish are still alive
If the octopus did make it into that tank it would not likely start on a killing spree right away. If the tank really is that close it is a possibility he made it in. There are a few published stories out there about octopuses at Public Aquariums going on excursions to other tanks. Although it sounds like your lid was pretty secure an octopus needs a gap or hole about the size of their eye to escape.