Rectangular building, maybe the size of 2 chicken coops side by side.
Fill with water or salt water depending on the fish being bred. Water quality as well as building material quality determines softcap of bred fish.
Insert roe, food, get fish. Must be at least 2 fish in the water to begin a continuous breeding cycle. Introduce new item "[x fish] minnow" for growing period. Maybe add arbitrary gender to fish to make male/female pairings more meaningful.
Only one type of fish can be bred per building? Or maybe each building has like 2-3 compartments for 2-3 different fish? I don't have really strong feelings towards this.
Takes lots of water. LOTS of water.
Water dirties over time (maybe quality gradually decreases?), can potentially kill fish, must be changed/cleaned at intervals.
Very small chance for green kelp to spawn in fisheries with quality matching current output.
Fish can be fed grains, seeds, meats, herbs, etc. (Swill, feed trough, normal stuff)
Maybe adding shrimp to the container helps keep it cleaner longer or something. Kind of like worms in compost bins.