Water never goes in one square on its own. It fills the tank’s whole row, and every row of the tank below. If that would put more water in some row or column than its number allows, the square is dry.
This is one of the Aquarium techniques. Each page shows its technique on the same 10×10 from the game, Hard level 16.
The rule
- Pick an open square, and see what water there would bring: the rest of its tank on that row, and every row of the tank below.
- For each row and column that water touches, add it to the water already there.
- If any of them would go over its number, the square is dry (and so is everything above it in the tank).
In a real puzzle
On the 10×10, it’s first needed at step 10. Column 2 is numbered 7 and already holds 6 water squares. Water in its row 3 would also fill the square beside it, and run down into row 4 of the column. That would make 8 in column 2, one too many. So row 3 of that tank is dry.
How often you’ll need it
On 17 of the 50 Easy levels, 41 Medium and every Hard one: 0.9 times per Easy level on average, 4.9 on Medium and 11.3 on Hard. On Hard levels, 55% of the time it’s about a column. It’s 6 of the 43 moves on the 10×10 here.