Yeah, even if the 650 is 10amp (chiller could be a lot though), you should be fine with a 15amp breaker.
I actually did the fill differently this time. When I got my 250 I filled it with RO, then mixed in the salt. Then dumped the sand in and let it clear.
This time I want to know how much water I actually have. It's a bit more work to fill 15 gal of RO and mix, then add to the tank, but it probably the best route.
Add sand & probably 15-20 gallons of water for the 650. Then get a large rock and pour the rest of the water onto the rock to help minimize wet sand craters & splashing.