The physics
Inside a bus that accelerates at a, Newton's laws need one extra force. Add a pseudo force −ma to every body. It points opposite to the acceleration.
T + mg + (−ma) = 0 ⇒ tan θ = a / g
You can also say it this way. Inside the bus, gravity acts like geff = g − a. A bob hangs along geff. A water surface lies at right angles to it. Buoyancy pushes against it.
Pressure at the tank floor is ρ g × the depth of water above it. The tilted surface makes one end deeper. So one floor corner feels ρ a L more than the other.
weight mg
tension T
pseudo −ma