Pool & room
The pool: size, depth and height
Big enough that luck won't do the job, and at a height your back will forgive.
The pool is the stage for everything else. Too small, and an animal can find the platform simply by swimming round at a fixed distance from the wall. Too big, and both the animals and the person reaching in to rescue them have a harder time. The original watermaze.org had two sections on this, and both still make sense today.
The Pool
The Morris water maze consists of a round tank (pool) of water. We think that 6ft (1.8m) to 2m diameter is good. We think that a 2 ft (0.6m) deep pool is good, with 1 ft (0.3m) of water. For mice 5-6ft (1.5-1.8m) diameter is good, and the same 2ft deep. We think that is good to place the pool on a 1-2ft base, to make working more convenient. The escape platform can be 4ins (10cms) in diameter. The platform should not be too close to the side, or the subject finds it just by circling around the pool-side. For rats the platform can be 1/4 in below the water. For mice it should be very close to the surface.
We are told that a good water temperature for rats is 26 deg C ±2 deg. Colder encourages activity, but can cause hypothermia, which impairs learning.
Original text: watermaze.org Part I, General Information. From original material by Richard Baker. Reproduced unchanged. Copyright Richard Baker© 2013. All Rights Reserved.
Size of pool & platform?
These are a matter of personal choice. One criterion, which sounds logical, is that the task should not be so easy that it can be achieved mostly by chance. I.e. the pool should not be too small, nor the platform too big. Also if the pool is too small, the platform might be close to the sidewall. In that case, just swimming around at a fixed distance from the sidewall will find the platform quickly. For adult rats a pool diameter if 1.8m (6ft) may be good, with a 10cms (4 ins) diameter platform. Depth ~ 60cms (2 ft). In the US we supply pools 4, 5 or 6ft diameter, 2 ft deep, with a 1ft water-depth. For mice a pool diameter of 1m to 1.5m is used, with platform diameter 5cms to 10 cms. Pool height ~30 cms, water depth ~ 25cms works. Mice are very aware of the room & operator, and a sidewall above water level could help block the line of sight. With all animals, it is vital that they can see the cues (on the lab walls), from the vicinity of the platform - otherwise they can't orientate. Nearly all labs use round platforms - this sounds good, as there is no preferred direction for approach. A few use square platforms - please note that HVS Water software is designed for round platforms.
Original text: watermaze.org Part I, General Information. From original material by Richard Baker. Reproduced unchanged. Copyright Richard Baker© 2013. All Rights Reserved.
Notice how the two sections agree on rats (about 1.8 m across) but give different figures for mice. Section one suggests 1.5 to 1.8 m; the later "Size of pool & platform?" note says 1 m to 1.5 m is used. Both are kept here as written. If you're choosing, our size guide puts the numbers side by side.
Questions people ask
How deep should the pool and the water be?
The water must be deep enough that the animal cannot touch the bottom, with enough wall above it to stop escapes. Rats: about 25–30 cm of water with about 25–30 cm of wall above. Mice: shallower, so they can see over the wall to the room cues. Your local ethics committee, licence conditions and vet always take precedence.
More detail
The pool should be deep enough that animals cannot stand or jump out, typically about 25–30 cm of water with a similar height of wall above it. Tank 50 cm deep with 25 cm of water [Vorhees & Williams 2024]; HVS: 60 cm wall with 30 cm water [HVS]. The water level should stop jumping out but not block the view of the room cues [Vorhees & Williams 2024].
Going deeper
A full 2 m pool at 30 cm holds about 0.9 t of water (illustrative), and over 1.5 t if over-filled [HVS].
Should I buy a pool or build my own?
Either buying or building can work, as long as the pool meets the specification. It needs the right diameter, a smooth, seam-free, non-reflective interior, a drain, a stable stand with locking castors, and safe electrics. Compare total cost including your time, the drain, stand and repairs. Your local ethics committee, licence conditions and vet always take precedence.
More detail
Either a bought or a home-built pool can work; what matters is that it meets the same specification, and the fair comparison is total cost, including staff time. The specification covers diameter for your species [Vorhees & Williams 2024]; uniform, seam-free interior; sloped drain; laminated polypropylene or similar [Vorhees & Williams 2024]; locking castors and safe working height [HVS]; RCD-protected electrics [HSE].
Going deeper
Compare like for like, including staff time, not purchase price alone. For each point, what to ask a supplier or plan for a DIY build: 1) Truly circular, with a stated internal diameter and depth (calibration and zones assume a circle; size sets difficulty [Vorhees & Williams 2024]): ask the diameter and roundness tolerance, or use a former and check roundness before finishing. 2) Seam-free, non-reflective, durable interior (seams act as cues and painted finishes can flake [Morris 1984]): ask the material, finish, colours and warranty, or allow for a gelcoat or plastic lining, curing time and repairs. 3) Load-rated stand with locking castors (a full pool can exceed 1.5 t, and movement changes calibration [HVS]): ask the rated load and lock type, or have the frame and floor loading checked. 4) Drain and fill (water is typically changed once or twice a week [Vorhees & Williams 2024]): ask the drain size, valve and hose route, or plan plumbing and floor-drain access. 5) Stable water temperature [HVS]: ask about heater options and thermostat accuracy, or use a heater rated for wet use on an RCD-protected supply. 6) Curtains and cue mounting: ask about track or frame options, or plan a ceiling track. 7) Access: large pools may not fit through doors [HVS]; ask about a sectional build, delivery and installation, or allow assembly space, sealing and welding. 8) Total cost: delivery, installation, spares and lead time, against materials, staff hours, rework and downtime.
Spotted a mistake, or know a better source? Tell us and we'll fix it and say so on our corrections page.