Still, there are also hidden costs. When millions of fish are crowded together, they generate a lot of waste. If they’re penned in shallow coastal pools, the solid waste turns into a thick slime on the seafloor, smothering all plants and animals. Nitrogen and phosphorus levels spike in surrounding waters, causing algal blooms, killing wild fish, and driving away tourists. Bred to grow faster and bigger, the farmed fish sometimes escape their enclosures and threaten indigenous species.