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?
Only once a year for ponds after a harvest and drain. I may even get away from that and just use rotenone after as many fish are removed as possible via seining.
2) What is the duty cycle of the recirc tanks? Constant I assume?
Not necessarily. Still working out the numbers. Kind of think contant but low flow inconjuction with the mechanical filters and biofilter may be the best way to go. Could get by with much much less flow than 45 gpm 24/7 if I can keep the water under 65 F. Well water is 51.6 F.
3) Any estimate as to what it is costing to run the present pump in either cost per hr or cost per day?
Last I checked the cost is about $100.00 per month to pump the well continuously. Believe it or not that still makes the sale of the big trout profitable at just two of them will cover the cost of one month of pumping and I sell about 100 +.
