I have a 25 gallon tank that has had the same fish in it for years. It's had 6 tetras, 1 red tail shark (3.5"), 1 small pleco (2") and 1 large pleco (8"). The newest fish are the tetras and they've been in there over a year. The oldest resident is the large pleco and he's been in the tank for 6 years. All of these fish have lived in perfect harmony. The plecos liked the bottom, the tetras the top and the shark liked it everywhere. They all went in all areas of the tank on a regular basis without issue though.
Last week we had a death in the school. The large pleco passed. I got him when he was only an inch long. What a cool dude he was. Anyways...
Shortly after the death I noticed a change in the sharks behavior. He started getting very territorial and aggresive towards the tetras. As soon as a tetra swims down to the middle of the tank the shark is on the tetra like stink on poop. He zooms around the tank at light speed chasing the tetra back up to the top and then leaves the tetra alone. He does this with the remaining pleco as well but not nearly as bad. The tetras are basically banned from the entire tank except the top one or two inches.
It looks like the shark, now being the biggest fish in the pond, has decided to start enforcing his rules on the others. Behavior he never displayed until the big pleco passed. So, is this normal? I'm half tempted to pull the shark out of the tank and set him up, all alone, in a 12 gallon tank I have but am not currently using.