In general, algae is not necessarily bad for aquarium fish. In fact, small amounts of algae can be beneficial as a natural food source for certain fish species, and can also help to improve water quality by absorbing excess nutrients in the water.
However, excessive algae growth can be a sign of poor water quality and can negatively impact the health of your fish. When algae grows out of control, it can reduce oxygen levels in the water and create an unsightly and potentially harmful environment for your fish and other aquatic life.
Additionally, certain types of algae can release toxins into the water that can harm fish and other aquatic life. For example, some species of blue-green algae can produce toxins that can cause skin irritation, respiratory distress, and other health issues in fish.
It's important to maintain a healthy balance of algae in your aquarium by controlling its growth through regular maintenance and good aquarium hygiene practices. This can include reducing the amount of light that the aquarium receives, limiting the amount of food that you feed your fish, and performing regular water changes to remove excess nutrients from the water.