The grid is divided into aquariums by bold walls. Every cell is either air or water.
Water obeys gravity: within an aquarium it settles to one flat level. Every aquarium cell at or below the water surface is water, and every cell above it is air.
Because water finds its level, all cells of an aquarium in the same row share the same state, and water never sits directly above air inside the same aquarium.
A number beside a row or column tells you exactly how many cells in that line are water.
A row or column number turns green when its target is met and red when it holds too much water.
The puzzle is solved when every aquarium is filled to one flat level and every clue is satisfied. Cells that break a rule turn red.
Within one aquarium a whole row is water or a whole row is air — a level is always flat. If any cell of an aquarium in a row is water, every cell of that aquarium in the same row is water too.
Example: An aquarium spans three cells across a row and one of them must be water. Fill all three of that aquarium’s cells in the row with water.
Inside an aquarium, water fills from the bottom. If a cell is water, every cell of that aquarium below it is water; if a cell is air, every cell of that aquarium above it is air.
Example: You place water in an aquarium cell. Mark every cell of that same aquarium directly below it as water as well.
A row or column clue of 0 is entirely air; a clue equal to the board width is entirely water. Fill those lines in first.
Example: A column labelled 0 contains no water, so mark every cell in that column as air.
Once a row or column holds its full number of water cells, every other cell in that line is air.
Example: A row has clue 3 and already holds three water cells. Mark the rest of the row as air.
Count how many water cells a line still needs, then count the cells that could still be water. If the numbers match, all of them are water.
Example: A row needs 2 more water cells and only 2 cells remain that are not yet air. Both must be water.
An aquarium can only be filled to one of a fixed set of levels — each distinct row it occupies, plus empty. Test which levels are still possible against the row and column clues.
Example: A tall aquarium in a column that needs just one more water cell can only be filled to its very bottom row.
Every water cell counts toward both a row clue and a column clue, and every aquarium spans several columns at once. A deduction in one direction often forces a level in another.
Example: Finishing a column’s water count can cap how high a wide aquarium may rise, which then settles its cells in the neighbouring columns.