I've been there - it's fascinating. On the Southern tip of Lake Windemere. There is a similar (and much bigger) equivalent in the Loire Valley in France. Absolutely incredible!
To fish or not to fish, that is the question. Think carefully before you answer...
Jack - That is normal for carp to sit mid water. It can be down to all sorts of things. The temperature difference in such a shallow tank will be extremely minimal.
When I was their I asked when they feed them, I wanted to see the way the fish fed up in the water.
The woman there said that they only feed the fish outside visiting times. But she did say that it is a sight to be seen, all the fish flying around, water everywhere. Sounds like a fish farm feeding session, just with fish to 15lb instead of 15oz.