Aboleths are CR 10 aberrations. They're basically "Cthulhu-fish" that can inflict terrible diseases on targets that turn them into fish people. Here are a few basic things you should know about their tactics.
Use phantasmal force to keep dangerous, low-intelligence targets away from the pool while the aboleth picks off the weaker foes.
Target low Wisdom targets at the water's edge with Enslave. After they are charmed, make them leave the area so the aboleth can focus on weaker foes.
Place the aboleth in a pool of water that is at least 40 feet deep. It can swim up to 20 feet on its turn, make its tentacle attacks, and then dive 20 feet back down into the water to stay safe.
The aboleth can use its grasping tide Lair Action to pull targets within 20 feet of the water's edge up to 20 feet into the water (for a total distance of 40 feet).
Art by Matias Lazaro.