Quote Originally Posted by mas985 View Post
I have the same thing going on but I believe that it is due to outgassing. The pump basket pressure is below atmospheric so any dissolved air will tend to outgass at a rate relative to the pressure difference. On high speed the flow rate is sufficient to expell any air bubbles that may form but on low speed the air tends to collect on the top of the pump basket albeit very slowly as in my case. I suspect those with waterfalls and spillovers experience this more than those with no way to increase the dissolved air in the water.

You can also see this occur when the pump is off but in the sun (or a sealed bottle in the sun). As the water heats up, it can hold less air and bubbles will start to form on the pump lid. I see this all the time in my spa pump when it is off.

However, if air collects quickly, it is likely a pump lid air leak as the lid does not seem to seal as well on low speed as it does on high speed when the suction is stronger.
Thanks! That really sounds about right for what is happening. I also note that tonight, now that it's cooler, there's no bubble at all on low speed, so I think the temp is in play, as per Mark's explanation.