So just for clarification. Everything was working perfectly fine before you installed the heat pump, correct?

Also, maybe I misunderstood your first post. The 3 PSI rise in the filter, was that due to the heat pump or something wrong with the filter?

It sounded like low speed was working ok with the heat pump but then something happened after than and now it doesn't work? I am trying to understand the history.

Low speed flow rates are going to be half of high speed, so you can't expect all that much out of low speed. While you don't have any lift with low speed (water feature is at same height as pump correct?) the extra head loss in the heater might be enough to reduce the flow rate to a level that is not all that useful.