A lot of the player homes you visit have fish in the ponds and streams around them. How do you stock your water with fish? I have a couple of ponds and a waterfall around my house and I'd love to stock them with fish so people can fish there.
This isn't something you do from your end. Fish can be found only in certain castles like the school houses (Fire House, Ice House, etc), bundle castles and some others. If you don't see fish in your castle, it is because that type of house simply doesn't have them.
This can change if the house is ever updated to include them, of course. But we might have a long wait if ever.
