What did I do wrong?
 

I read the manual a few times but ???
I wanted to slow down the RD 80W Speedy for a water change.
Well the speed dropped really fast & then the pump shut down,
dn showed on the controller.
No problem, seemed better than just pulling the plug.
I have been able to restart & control the flow without incident, BUT--
I can no longer use a GFCI outlet.
The outlet is no longer usable after playing around with different ways to run the pump. A new plug does not work either so I now use a regular grounded 3 prong outlet.
Any thoughts?

I don’t think you did anything wrong. Some GFCI’s and variable speed pumps often don’t play nice. Even though it might have worked for a bit, variable speed pumps create differential voltage which can trip many GFCI’s. If you do a google search on “variable speed pump trips GFCI”, you will find lots of stories about this scenario. Also, GFCI’s become more sensitive over time. I personally will not run GFCI’s on my system for this reason. I have 2 GFCI’s in my kitchen. One will trip from different pumps and the other will not. There are commercial grade GFCI’s that have a 20ma trip sensitivity which usually are fine as are AFCI’s and or ELCB breakers which are better suited for this. GFCI breakers are often a bit better as well but most run GFCI recepticals which are really sensivive. Siemens also makes good GFCI breakers that are less sensitive to variable speed pumps and are much better suited for our use than GFCI recepticals IMO and experience.