I was just going through the rabbitry switching out and cleaning water bottles today, and thought I'd write a little bit about that since it is a job everyone has to do from time to time, especially in the heat of summer with lots of litters and full cages to deal with. Even if you don't use water bottles and have a watering system instead, it is still necessary to flush it out from time to time to eliminate mold and other undesirable bacteria that build up in a wet environment continually.
My favorite bottles in the absolute whole world are Lixits:-). Not only do they have a really simple hanging wire that eliminates complex brackets, but the bottles themselves are wide mouthed and tilted on a 45 degree angle into the cage, making them easy to clean and position for perfect access.
One easy, no-scrub way to clean a water bottle is to put a few drops of bleach into it, fill it halfway up with water, put the cap on, and then leave it sitting on the counter on it's side until the mold and dirt 'melt' off. You will need to clean the outside of the bottle by hand after that, or simply pop the whole thing in the dishwasher to sterilize.
If you have a limited amount of bottles and need to wash them by hand fast in order to return them to the cage right away, just grab a sponge with a little dish liquid on it, wipe down and rinse the outside of it, and then take a bottle brush (the kind with a brush at either end---large and small sized), and scrub the inside thoroughly.
Sometimes the brush is not enough to get around the mouth of the bottle and the little nooks and crannies around it, so at that point I take a sponge on a stick (or just a really soft sponge that I can squeeze part-way into the bottle), and scrub it out again (with lots of rinsing). I then wash the cap by hand because it is tough to get that totally clean in the dishwasher, and I use the small end of the bottle brush to push in and out of the zip tube a few times, because lots of slime and grimy stuff can hide out in hard-to-reach places like that:(. After this (if there is time), I put all bottles in the dishwasher on the top rack, and all caps in the utensil basket to get everything sterilized and ready to use.
There is nothing wrong with skipping bottles completely in favor of water dishes, and lots of people prefer those year round because they are easier to clean and much less complicated to use. With wool breeds, however, I find that dishes tend to be unsanitary and a downright disaster for long coats, especially when someone somewhere decides to stand in the dish in order to cool off and cakes the wool on their chest and dewlap into a solid, unremovable matt:(.
Anyhow, so this is a little bit about how I keep water bottles clean, though people everywhere may have alternative methods. I have maxed out my supply of feed dishes and water bottles completely with all the babies that are growing around here, so I am now doing cleaning and maintenance 24/7, and can't wait til the weaning is finally done so I can get things down to a manageable level again, LOL:).
Have a wonderful week!