I live in Northeast Philadelphia as well and I am always in the King of Prussia area. Here is a little break down for you.
Pets Plus is probably the best in Philadelphia for saltwater fish and coral.
East Coast Aquatics, down by the airport, does not have much of a supply, but they sometimes get rare and nice stuff in.
The Frazer Zoo, 10 minutes from KOP, has excellent saltwater stock.
For supplies, That Fish Place, in Lancaster, is the only way to go. They are by far the cheapest in Southeastern PA. They are about 45 minutes to an hour from KOP. It is definitely worth it to make a trip up there every so often and stock up on supplies. They are just like a huge warehouse.