I never did a Morris water maze experiment, but I did a forced swimming test with rats, and it is supposed that you change the water after each animal because rats urinate and defecate in it, and it's not only dirty, but the smells of urina and faeces are the social behavior stimuli, and you don't need these stimuli in your experiment. Practically, people may change water after 3 or 4 rats if it's hard to refill the reservoirs, but it's undesirable and may distort your results. And definitely you can't store this water for several days.

I suppose that you may find tempera paint in any art supplies store.

You can use milk, but it will need to be shifted daily-- we run many rats, 50 or more in one set of water ,made opaque with milk. It is not the same as the forced swimming task--much larger volume of water and, after the first day, much less stress--there is an escape after all and the duration of each trial is much shorter. This water really only needs to be changed daily because the powdered milk will sour. You can purchase large bags of powdered milk in bulk and probably need to only add about 3-4 cups for a 5' pool about 2' deep. The paint is better and can last several days without needing to be changed. For paint, you will also need to stir it each day as it may settle to the bottom. Paint is also better if you have white rats-- use black tempera. I pick up paint at Michael's or A.C. Moore. You can probably also find it at most arts/crafts stores but I would recommend a bigger chain because it will be much less expensive. Walmart occassionally carries it too. Probably amazon as well. Whatever we use (now we have a tracker that doesn't require masking so we use plain water and it can last for at least a full week of testing)-- we always fish out all feces to prevent build up in the pool and it can be distracting to rats if you leave it floating on top of the water. Good luck!

We get out paint from a teacher's store. We have to use black paint because our rats are white.

Even without milk we change our water daily (to get a better control on the temperature). Also remember the animals will dirty this water very quickly.

An alternative to coloured water is to use a transparent escape platform (e.g., plexiglass). You will need to make sure the top surface is not too slippery to climb. This is assuming you do not need to change the water colour because of your rats' fur. 

Also, a slight problem with using milk powder is that the rats will smell like sour milk after a few days...

Provided you stir the water each day before testing and that you remove floating faeces after each trial, you should be good to go for a good week of testing even with rats. Any kind of nontoxic paint would do the trick, the kind that is intended for children especially. Those are considered safe for child so they are certainly expected to be decent for rats (but keep in mind you need to stir daily before getting started). Having some kind of plexiglass platform is definitively the way to go but I would still recommand to use tainted water.

In our lab we use latex. Is nontoxic and they cannot see the platform. For more information see related research from any Chamizo. V.D. publications (you can find also in Research Gate). Hope it helps!

I totally agree with Mathieu Wolff. This is what we have been using for many years, using rats. You may check our published work for further detail. In most of ou papers the first Author is Kanit, L and I (Pogun, S) am the last author.

Thanks a lot for helping me ....

Just a reminder: To contro water temperature, we have a heating system with a thermostat. We heat the water with it (we put it in the tank), when we reach the desired temperature, we take it out. 

You can use milk powder or black dyes that made of soot. The soot does not paint rats.

Dr. Uri Shalev,  Can I use black colored platform for a black colored tank, to replace colored water.?

can i use black tempera paint for white rats . will the black tempera paint color the fur of rats ?

I'm afraid I have never used those colours so I do not have an answer regarding colour on the fur.  You can certainly use a black platform in a black tank, but again, a transparent material would work with both white and black background

hi Senthilmurugan,

in our lab, we use tank painted in matte black color ( non-glossy), and a black platform with white rats, when the transparent plexiglass  was used , it shined in the "clear" water (reflected the light used above the tank). the clear water , white rat and black tank with a black platform, worked seamlessly, and we published our results.

hope this helps you to decide.  

good luck

You can use milk powder or soot.

good luck