Aquarium Logic Puzzle

Aquarium is a logic puzzle about filling tanks with water. The board is split into tanks, and numbers outside the grid say how many cells in each row and each column are water. You can play grids from 6×6 up to 10×10, and every puzzle here has exactly one water set that you can reach by logic, with no guessing. People also look this up as an aquarium logic puzzle, or as water fun.

Rules

The board is an n×n grid divided into tanks. Thick lines are the tank walls. Thin lines are the cells inside a tank. A number above a column, or to the left of a row, is how many cells in that line are water. A blank slot means that line has no fixed count. It is not the same as 0. A printed 0 means every cell in the line stays empty.

Fill a cell with water, or leave it empty. A cross is a mark for a cell you already know is empty. Crosses are optional. The puzzle does not ask you to cross every empty cell.

Water in one tank has a single level. If a row of that tank is wet, every cell of the tank in that row is water. The wet rows are the bottom rows of the tank. Water cannot float above a cell you already know is empty in the same tank.

The puzzle is solved when the water cells are that unique set. Empty cells may still be blank. You do not have to place a cross on every empty cell.

Controls

Choose Water or Cross under the board. Tap a cell, or drag across cells, to place that mark. Tap a cell that already has the mark to clear it. A cell that has the other mark takes the new one. A drag follows the first cell: it places the mark, or it clears when that cell already has it. With a mouse, right-click places a cross, or clears a cross that is already there.

On a keyboard, the arrow keys move around the grid. Space or Enter places the selected mark, or clears it when the cell already has that mark. W or 1 places water. X, C, E, D, or 2 places a cross. M switches Water and Cross. Backspace, Delete, or 0 clears the cell.

An outside number turns teal when the water count matches and every cell in the line is already marked. A line of 0 stays plain while any cell is still blank, so an untouched board does not look finished. A number turns red when the line already has too much water, or when the cells still open cannot reach the number. A tank turns red when its water is uneven or floating. Press Hint for one step the rules already force, with a sentence saying why. Press Check when you want the board checked against the rules. Undo and Redo step through your moves, and Reset clears the board. When the water cells are the unique set, the board turns teal.

Techniques

Easy grids follow from one line or one tank at a time:

• Line zero: a 0 leaves every open cell in the line empty.
• Line full: a number that matches the length of the line fills those cells with water.
• Line quota: the number forces the cells that are still open.
• Single: a one-cell tank is forced by the line’s number.
• Wide empty: a tank wider than the row number leaves that run empty.
• Stack cap: filling a cell would put more water in the column than the number allows.

Medium and hard grids look at every way a row can make its number, then at the water height of one tank. The hardest step tries the other heights of a tank and keeps the height that does not break a clue.

Beginner tips

Start with the 0 lines. They clear a whole row or column, and those empty cells often force a tank beside them to stay empty as well.

When a line already has its number, cross the rest of that line before you look for a tank height. Then check that water in each tank is one flat level on the bottom.

If you are new to Aquarium, start with the daily 6×6 easy grid, or another 6×6 easy grid, and move up once the outside numbers and the water level feel natural.