I went down to Cove Brook to get some peace and quiet but nature thought otherwise! First it was nice to see a fish swim by, over a foot long with a wide round head, which I was very surprised to see in the Brook, likely a pike or maybe a cat fish. The foxes were all out and then one caught something down by the water, it was horrible, whatever it was, was screaming and screaming desperately for 10-15 minutes then went quiet, I saw the fox but it hid whatever it had caught. I thought I liked nature BUT...... I wonder if it was maybe a mallard or a moorhen, I'm also worried it could have had babies but the grass is long there, I couldn't get near enough to see. I must have seen 4 foxes there tonight, I do love foxes, my allotment foxes are really lovely, they mate for life and really look after each other and are a close pair, but I do wonder what it caught and I hope they don't eat all the ducks :-( Nature red in tooth and claw.
As you say, nature red in tooth and claw, or as often said, hunt and be hunted.
I can't help you with the fish ID, and certainly not without a photo, though someone might be able to. As for the foxes, there's a good chance they will have had a mallard, or two. Their cubs are now growing fast and many almost adult fox size, so will have the food requirement of an adult.
There are large species of Carp in the UK and there are also wild Catfish which can grow very big...