The grid is divided into tanks by thick walls. Some tanks hold water, some are empty, some are partly full.
Water behaves like water: it settles at the bottom of a tank and its surface is flat. So if one square of a tank is filled, every square of that tank at the same height or lower is filled too.
The numbers outside the grid say how many filled squares each row and column has. A row of 0 is dry right across, and a tank that reaches into a row fills all of its squares in that row together.
Fill tanks with water that lies flat and settles at the bottom, so every row and column has the right number of filled squares.
The full guide, with a worked example and common mistakes: How to play Water Tanks.
Yes — empty, partly full and full are all fine.
No — each tank is separate, with its own level.
Yes — every puzzle here is checked to have exactly one solution.
Open the Water Tanks Solver, draw the tanks, type the counts and press Solve.