Sand & Filter ?

[Teelie]

Rotting material like food or waste gets buried then bacteria eat that and produce their own waste which forms the bubbles. It usually happens in more compacted sand (which happens in deeper sand) that allows the bubbles to form large enough to see and give people scares that it'll hurt their fish when they are released. The smell can be foul but it's really the same physics as a bubble wand. All it does is break the surface and allow gas exchange.

 

[Matty P]

Rotting material like food or waste gets buried then bacteria eat that and produce their own waste which forms the bubbles. It usually happens in more compacted sand (which happens in deeper sand) that allows the bubbles to form large enough to see and give people scares that it'll hurt their fish when they are released. The smell can be foul but it's really the same physics as a bubble wand. All it does is break the surface and allow gas exchange.

Thanks Teelie