several reasons why its bad.

- Bread doesn't have much nutritional value (especially the kind of white "bread" that is common in the US for example)

- It fills up their stomach fast so they don't forage for foods they would naturally eat

- Throwing bread at one place can lead to overcrowding (which is what happened in your great fishing spot) which stresses the animals

- They also shit a lot in one place, which leads to a bunch of problems

- Uneaten bread causes algae growth which leads to diseases and pests

best to feed them peas like in this neat app or I also heard that cut up frozen grapes are a good choice.