Here's why:
* Size: Most rainbow fish species are too large to effectively consume the tiny plankton.
* Mouth Structure: Their mouths are not adapted for filter feeding, which is necessary for capturing and consuming plankton.
* Diet: They primarily feed on insects, crustaceans, algae, and other small invertebrates, which are larger than plankton.
However, they may occasionally consume some zooplankton, especially if it's available in their environment. But it's not a significant part of their diet.
In the aquarium setting, you can supplement their diet with commercially available plankton flakes or tablets, but it's not essential for their health and well-being.
Therefore, while tropical rainbow fish may encounter some plankton, they don't rely on it as a primary food source.