I've just been round to exdh's to feed and clean out the guinea pig as he's away (US for 2 weeks lucky sod )
Anyway, the water bottle was very green inside. I was thinking of buying a new one for him when I remembered a How Clean is your house tip for cleaning vases and thought it would probably do for this. You put rice and vinegar in and shake. I found some rice and malt vinegar in the cupboard. They use white but I thought I'd try it anyway. I put my finger over the end and gave it a good shake. It worked a treat, came up like new. I thoroughly rinsed it and hey presto a lovely clean bottle