Play Aquarium Online

One decision per tank, not one per square

An Aquarium board looks like a grid of independent squares, but the thick borders carve it into a handful of tanks, and each tank holds exactly one decision: how high its water sits. Once a level is chosen, physics does the rest — every square of that tank at or below the level is wet, everything above is dry. A 10×10 board has a hundred squares but usually only twelve to sixteen tanks, so the puzzle is far smaller than it first appears. The numbers along the top and left edges say how many wet squares each column and row must finish with.

Pouring, draining and marking in this version

This implementation does the physics bookkeeping for you. Selecting a dry square pours water into its tank up to that line — the square you chose becomes the surface, and everything beneath it in the tank fills at once. Selecting the surface row drains it by one level, and selecting deeper water pulls the surface down to that line. You never have to paint squares one at a time, and you can never create a floating puddle that breaks gravity.

When you have proved a square must stay dry, record it: right-click on a mouse, or switch on Mark mode to place × marks by tapping. Marks are notes for you rather than answers, and if water later rises over a marked square the mark is washed away automatically. On a keyboard, the arrow keys move around the grid, Enter or Space acts on the selected square, X toggles a mark, and Delete drains the selected tank completely.

A tank is an all-or-nothing offer to each row

The most useful way to read the board is row by row, treating each tank as an offer. A tank that owns three squares in a row will put either all three or none of them into that row's count — there is no way to take just one. So a row clue is really asking: which combination of the offers crossing this row adds up to the target?

That framing turns wide tanks into levers. Suppose a row has a clue of 2 and one tank owns four squares there. Accepting that tank's offer would put four wet squares into a row that can only take two, so the offer must be refused: the tank's surface sits below that row, and every square it owns in higher rows is dry as well. One comparison has just dried a whole region of the board.

Zeros drain upwards, full rows flood downwards

The two extreme clues resolve fastest, and they cascade in opposite directions. A row clue of 0 means every tank crossing that row is dry there — and because water fills from the bottom, each of those tanks is also dry in every row above. A single zero can empty a third of the board.

A row clue equal to the full board width floods the other way. Every tank crossing the row is wet at that height, and gravity drags each of them down: all of their squares in lower rows fill too. After one full row, check the rows beneath it — their counts are often complete or nearly complete, which exposes the next deduction immediately. The same logic applies vertically: a column clue of 0 dries the whole column, and each dried square dries everything above it in its own tank.

Floors and ceilings for every line

Between the extremes, run a floor-and-ceiling count. For any row, the floor is what it holds if every undecided tank stays as dry as it still can; the ceiling is what it holds if every undecided tank fills as high as it can. Whenever the ceiling equals the clue, every undecided tank crossing the line must take its maximum. Whenever the floor equals the clue, they must all take their minimum.

These squeezes appear constantly near the end of a solve. A column needing 5 with confirmed water on 3 squares and exactly one undecided tank offering 2 in that column is finished — the offer must be accepted, and accepting it may fix that tank's level for the whole board. A column that crosses a tall, one-square-wide tank works like a dipstick: the number of wet squares it contributes equals the tank's depth directly, so the column clue often pins the level with no other help.

Awkwardly shaped tanks are the real difficulty

Rectangular tanks behave. The tricky ones bend: an L on its side, a U whose two arms rise separately, an S that snakes across four rows. The rule to hold on to is that a tank shares one water level across its entire footprint, however far apart its parts sit. If the left arm of a U is wet at some height, the right arm is wet at that height too — even when a different tank occupies the squares between them. The surface can look broken on screen while still being one legal level.

Bent tanks also make offers that change size from row to row. A tank might own one square in its top row and four in its bottom row, so raising its level by one step can swing a row count by four. When a clue seems impossible to reach, look for the tank whose offer jumps like this — the jump is usually the deduction.

Board sizes and what difficulty actually changes

Every board this page generates is checked by a solver so that exactly one arrangement of water satisfies all the clues. Difficulty does not change the rules; it changes the tanks. Easy boards use compact, chunky tanks whose offers are simple to compare. Medium boards use larger tanks and are chosen to need longer chains of line counting. Hard boards use the largest, most winding tanks, and the generator keeps the candidate that resists straightforward line counting the longest — expect to lean on floors, ceilings and the shape of individual tanks rather than single-line arithmetic.

Feedback, checking and recovery

The edge clues track your progress live. A clue whose line currently holds the right number of wet squares is struck through and dimmed; a clue whose line holds too many turns red with a wavy underline. The Water counter above the board shows how many squares are wet against the total the puzzle needs, which is a quick way to sense whether you are under- or over-pouring overall.

Check compares each tank's current level with the board's single solution and outlines any tank that is wrong, without saying what the level should be. Hint goes one step further and sets one incorrect tank to its correct level. Undo steps back through pours, drains and marks alike; Reset drains every tank and clears your marks; Solution reveals the finished board and stops the timer. The puzzle is complete the moment every row and column count matches — the win is detected from the counts themselves.

Aquarium beside Thermometers, Tents and Battleship

Several puzzles on this site hand you row and column counts and ask you to fill the grid, but each fills differently, and the differences decide how it solves.

Thermometers is the closest relative: swap "fills from the bulb along its own shape" for "fills from the bottom of the board" and you have moved from one puzzle to the other. The global direction is what gives Aquarium its cascades — in Thermometers a filled cell only tells you about one shape, while in Aquarium a filled cell tells you about every lower row of its tank.

Leaks that sink a solve

• Reading a tank locally. Pouring into one arm of a bent tank raises the far arm too. Before accepting a level, glance across the tank's full width for rows it will disturb elsewhere.
• Trusting the picture over the borders. Two patches of water at the same height may or may not be the same tank. The thick lines are the truth; the colouring is only the consequence.
• Solving rows and ignoring columns. Row logic tends to run out midway. The column clues slice tanks vertically and often hold the deduction the rows cannot see, especially through tall narrow tanks.
• Forgetting the offer changed. After any pour, nearby clue totals have moved. Re-count the floors and ceilings of the lines you touched before hunting elsewhere.
• Guessing levels to escape a stall. On hard boards a stall usually means one bent tank hides the key. Compare its per-row offers against the tightest crossing clue instead of pouring speculatively — and if you have already guessed, Undo back to certainty rather than patching around it.

Rules were cross-checked against the rule statement on puzzle-aquarium.com, which popularised the format online: one water level per aquarium, level across its full width, with edge numbers counting filled cells. Board sizes and the pour-and-drain control scheme are specific to this Glorified Calculator version.

Continue with another counting puzzle