That alpha 250 is WAY too big. The necks on one skimmers are huge, thus requiring a massive massive fish load.
I run the Bubble King double cone 200 on my 240 with 30 fish in it and it doesn't eve break a sweat to keep up. The alpha 250 is even a larger skimmer. These skimmers are conservative in their ratings. So that 450 gallon is probably a heavy load. Maybe 50+ fish. Something to consider.
As stated, check out the for sale forums for some good deals.
Corey