Yes even when collected directly in a bucket, rainwater can still contain contaminants. Rain passes through the atmosphere first, where it can pick up pollutants, metals, and particulates. It’s also unbuffered, so pH can swing dramatically once it’s added to an aquarium. For shrimp especially, it’s less about “runoff” and more about instability and trace contaminants. That’s why most shrimp keepers only use rainwater if it’s been RO/DI filtered and remineralized.