I’d pour some water and white distilled vinegar in it, shake it around, and repeat. Let it soak would be good as well. Best solution is get a pipe cleaner to try and scrub the sides.

The watered down vinegar (larger ratio of water) is the rabbit-safe cleaner a lot of people use. If you rinse thoroughly, the scent shouldn’t be left behind. He won’t get sick from exposure to vinegar smell though— Wick smells it all the time, as do I when I clean.

Apple cider vinegar is still acidic, so potentially. I’d spend a lot of time with either vinegars, because green growth is worriesome and you definitely don’t want any of that in the bottle.

In the future (unsure of your practices), I always dump and rinse out Wick’s bottle daily— I never just refill it. He doesn’t use a bottle anymore though.

I highly recommend replacing the bottle with a water dish instead. It’s much easier to clean and it’s easier for rabbits to drink out of = They will drink more water.

Tbh if it’s got green stuff on it I’d just dump it and get a new one. Even better, use a bowl. Far more hygienic.

You could use a diluted vinegar solution as Wick suggested. I use it to clean their litter boxes and they aren’t bothered by the smell, however I’m not sure of how it might make the water taste? I would think that it might be harder to get the taste out if it’s a plastic bottle, my bottles are glass and I’ve just rinsed them out because they’ve never been visibly dirty.

I’m with sarah though, personally I would probably just buy a new one. But I understand not everyone has the spare cash. A bowl is a good idea, but I work full time so I still keep the bottle because I’m afraid that someone will dump the bowl over and be left with no water until I get home.

I tried the bowl too! They seemed confused by it and would end up stepping in it at some point and then freaking out and knocking it over… hahaha

You can use rice to help scrape inside of the bottle to clean it.

I have two methods to clean a bunny bottle. One is put some rice and detergent into the bottle and then shake the bottle to scrub the inside. The other is using a fitted size bottle brush. For example, you can find a brush set that contains different brushes. Use the sponge brush to scrub the inside or outside of the bottle. Use a bristle brush to scrape crannies. 

I don’t use bottles anymore, but was also going to recommend shaking rice with a bit of water, it’s really effective.