Aquarium vs Thermometers: Two Fill-to-Level Logic Puzzles

If you enjoy the aquarium puzzle, there is one other logic puzzle you will recognise instantly: Thermometers. The two are the closest of cousins, both built on the same satisfying idea of filling cells up to a level and matching the count to numbers along the edges of the grid. Play one and the other feels familiar within seconds. Yet they are not the same puzzle, and the difference, tubes versus tanks, changes how each one feels to solve. This guide compares Aquarium vs Thermometers, shows exactly where they overlap and where they part ways, and helps you appreciate what makes each one tick. Fancy the water puzzle right now? Play an aquarium puzzle.

The shared idea: fill to a level

Both puzzles belong to a small family you could call "fill-to-level" logic puzzles. In both, you decide how far to fill each shape, and numbers around the grid tell you how many cells in each row and column end up filled. In both, the fill is directional, it rises from one end rather than appearing in the middle, and in both you never guess: careful counting and the fill rule always get you there. If you love that "raise the level to match the numbers" feeling, you will enjoy them equally.

The difference is in what you are filling.

How Thermometers works

In a Thermometers puzzle, the grid is packed with thermometer shapes: narrow, one-cell-wide tubes, each with a rounded bulb at one end. You fill each thermometer with mercury starting from the bulb and moving along the tube, just like a real thermometer rising. The crucial rule is that mercury cannot skip: you can only fill a segment if every segment between it and the bulb is already filled. The row and column clues tell you the total number of filled cells in each line. Because each thermometer is a single linear path, its fill is a simple "how far from the bulb does it rise?"

How Aquarium works

In an Aquarium puzzle, the grid is divided into irregular two-dimensional regions called aquariums, and you fill them with water. Water rises from the bottom under gravity and, crucially, it sits level across the full width of the region at every height. So an aquarium is not a one-cell tube, it is a container that can be several cells wide, and when its water level rises, it fills every column the region spans at that height. The row and column clues again give the filled counts. (For the full rules, see our what is an aquarium puzzle guide.)

The key difference: tubes vs tanks

Here is the heart of the comparison. Thermometers fill along a one-dimensional path; aquariums fill across a two-dimensional region.

In a thermometer, raising the level affects only that single tube, one cell at a time. In an aquarium, raising the water level affects every cell across the region's width at that height at once. That width matters enormously. A decision about one aquarium's level can change the filled count in several columns simultaneously, which ripples into other rows and aquariums. That cross-column dependency is exactly what gives Aquarium its richer, more interconnected logic. Thermometers is the cleaner, more linear puzzle; Aquarium is the chunkier, more spatial one.

Side by side

	Thermometers	Aquarium
What you fill	One-cell-wide tubes	Irregular multi-cell regions
Fill direction	From the bulb along the tube	From the bottom up, by gravity
The level rule	Mercury can't skip a segment	Water is level across the full width
Clues	Filled count per row and column	Filled count per row and column
Shape of logic	Linear (one tube at a time)	Spatial (width spans many columns)
Feel	Clean and tidy	Chunkier and more interconnected

Which is harder?

Neither is objectively harder; they stress slightly different skills. Thermometers tends to feel more orderly, because each tube is independent until the row and column counts tie them together. Aquarium tends to feel more entangled, because the width of each region links many columns at once, so a single water level can have wide consequences. Solvers who enjoy clean, almost mechanical deduction often love Thermometers, while those who enjoy a more spatial, "see the whole grid breathe" puzzle gravitate to Aquarium. Both are pure logic with a single solution.

Which should you play?

If you have only met one of them, try the other, because the skills transfer almost completely. Reach for Thermometers when you want a tidy, linear fill puzzle, and Aquarium when you want the same fill-to-level satisfaction with a richer, more two-dimensional twist. Aquarium's water metaphor also makes it especially welcoming to newcomers, as we explain in why aquarium is the most beginner-friendly logic puzzle.

The best way to feel the difference is to fill a few aquariums yourself. Play an aquarium puzzle now, or learn the rules first.

Frequently asked questions

What is the difference between Aquarium and Thermometers?

Both are fill-to-level logic puzzles with row and column count clues, but Thermometers has you fill one-cell-wide tubes from the bulb upward, while Aquarium has you fill irregular two-dimensional regions with water that stays level across the full width. The tube is linear; the aquarium spans multiple columns, which makes Aquarium's logic more interconnected.

Are Aquarium and Thermometers the same puzzle?

No, but they are close cousins. They share the core idea of filling shapes to a level and matching row and column counts, so the skills carry over. The difference is the shape you fill: Thermometers uses linear tubes, while Aquarium uses wider irregular regions where water is level across the whole width.

Is Aquarium harder than Thermometers?

Neither is objectively harder; they challenge you differently. Thermometers is more linear and orderly, with each tube fairly independent. Aquarium is more spatial and interconnected, because a single water level fills every column the region spans, so decisions ripple across the grid. Both are solvable by pure logic with no guessing.

What are fill-to-level puzzles?

Fill-to-level puzzles are logic puzzles where you fill shapes up to a chosen height and match the number of filled cells to clues along the rows and columns. Aquarium and Thermometers are the best-known examples: Aquarium fills regions with water, and Thermometers fills tubes with mercury, both rising from one end.