Probably about 20 but the deep end is about 4 feet deep so there is plenty of room. They are less bothered about feeding as the cold weather comes in and they rest a little deeper for the winter. There are two new ones now so I videoed their new home for the people I bought them from. Best pets, no walkies, all complaints very quietly expressed. No idea what they think about, nor do they care about what I think. But they are loved!
This probably comes from an earlier experience which may or may not be remembered. A session of Hypnotherapy could resolve this. Most phobias have a root from early life. Warm wishes Den :)
Try a local Hypnotherapist, this along with many phobias, responds extremely well usually. It should only require one or at most two, sessions. Any more is the therapists ego! NLP has also been seen to be useful. Warm wishes Den Clare :)