The rule of thumb is to filter the water at the rate of once every hour for smaller ponds. As pond size increases over 6000-7000 gallons this can be some what decreased to about once every two hours. Of course a lot of factors, especially fish load can affect this.
In your case, since you'll be over 10,000 gallons, i would suggest that you maybe you look at using two pumps in tandem, say something in the 6000-7000 gallon range. That way, if you ever have a pump issue, you can take one pump out of service for a 24 some time and still have some minimum filtration.
Also, if you do that, make sure you have enough valve shut-offs so you can actually isolate each pump for removal without shutting off the other.
In addition, if at all possible, run two electrical circuits on different breakers and plug one pump into each circuit. That way, should the GFI go, or the breaker pop, you'll still have one pump running on the other circuit.
Also, plumb the returns so you can divert the return to either the waterfall or the pond, or a mixture of the two. That way you can have full control of the waterfall effect and can divert excess water back to the pond. Or if need be, send it all to the pond and shutdown the water fall for repair or maintenance.
Craig
