It is not widely known that the first fish farmers in Britain were monks. It is easy to see why. Their diet involved eating fish on fast days, which included all of Lent and several other days a year. This was fine if they dwelled near the sea, but if you lived inland you only had rivers, and other people wanted their share of the fishing. But monasteries had land, and so many of them developed their own fish farms for freshwater fish.
Monks ate carp, which can grow big and fat, so could feed many monks. Carp have the advantage of being the aquatic equivalent of pigs, as they are omnivores and therefore eat anything, including scraps, and unlike pigs are not likely to bite human; and they do not smell or make those who work with them smell as well. This made them ideal for monks who want to be clean for their regular prayer duties.
The technique was simple. The monks dug two large ponds about four feet deep and ensured that there was a ramp going down into each. One of them was filled with water and stocked with carp, which were allowed to feed happily there. The second was left empty, and cattle taken down the ramp and allowed to graze in it. The method behind this was that the cattle would manure the ground and enrich it. Next year the first pond was drained, Enough fish were taken from the pond to feed the monks. but a breeding stock was kept, and the second filled, with the surviving fish transferred there. Here is where the manure mattered. It fostered the growth of pond weed, on which pond life could flourish. The carp would eat the pond life, along with any feed the monks put into the pond.The cycle would be repeated every year.
Here is where the third pond comes into play. This was known as the stew pond, and it was deliberately kept as clean as possible and not manured. This is where the fish were kept prior to eating. The reason for this is that carp can take on a muddy taste if they are kept in muddy water, so to purify the flesh of the muddy taste they were kept in clean water for a week or two. During this time they would only feed on insects that came to the surface of the water.
The monstrous act of vandalism that we miscall the English Reformation drove the monks from their homes and ended much of the good that they did. Fish farms were part of the loss. You can occasionally see the sedimented up remnants of monastic ponds in some of the estates that were stolen from the monks. They are small depressions in the ground prone to weediness and flooding when it rains, always near old monastic sites.