I did a little research into the variable speed pumps and from what I was told the longevity/reliability are not there just yet. Not sure if your existing pump would function with a variable frequency drive to vary speed. This is at the limits of my knowledge, but something I might investigate if in your shoes. Freq drives are spendy, but if you shop you can find some deals.
From what I understand, the best way to run a pump is constant. The starting and stopping are what is hardest on them and also consumes considerable juice to start.
The questions I would have is:
1) What is the duty cycle of the pond filling project? Is it really intermittent?
2) What is the duty cycle of the recirc tanks? Constant I assume?
3) Any estimate as to what it is costing to run the present pump in either cost per hr or cost per day?
