[AquaticLife] aerators
If you mean an airstone, bubble wand or bubble decoration in a tank, it is mostly decorative but the bubbles “popping” on the surface do provide additional surface agitation which does allow for gas exchange of O2 into the water and CO2, nitrogenous gases, etc. out of the water.
If you have an HOB filter that waterfalls back into the tank or if you have your return from any other filter that causes surface agitation, this will also do the job and usually provides a lot more agitation and in that case, the airstone goes back to being mostly decorative.
Lenny Vasbinder Fish Blog - http://GoldLenny.blogspot.com
Category: Philippines Internet Koi Society
