Here's why it's confusing:
* Bubble stones are typically used in air pumps to create a stream of bubbles in aquariums. This is primarily for aeration, not filtration.
* Filters themselves have various components:
* Mechanical filters capture larger debris.
* Biological filters house beneficial bacteria that break down waste.
* Chemical filters remove harmful substances from the water.
Possible interpretations:
* Someone might be referring to an air stone used in conjunction with a filter. Some filters might have an air pump attachment that allows you to connect an air stone, creating bubbles for aeration.
* It could be a colloquial term for a certain type of filter media that creates a bubbly effect. This would be highly unusual, as the focus of filter media is usually on efficient filtration, not aesthetics.
To clarify the meaning, you'll need more context:
* What type of filter are they talking about?
* What is the intended function of this "bubble stone"?
Providing more information will help determine what they actually mean by "bubble stone".