As a Delawarean, here's my favorites and my perspective on each (in brackets). These are all in New Castle County, which is Delaware's norternmost county. None of these have "feature tanks", they're all your typical livestock setups:
Delaware's Premium Aquatics (not far from University of Delaware)
http://www.premiumaquaticsde.com/
5 Chestnut Hill Plaza
Newark, DE 19713 (302) 224-3474
(Clean store & tanks, decent prices, knowledgeable staff, healthy livestock which can run a bit thin in quantity at times, but they don't have a lot of equipment or supplies beyond the basic reef stuff)
Williams Greenbank Aquarium (no web site)
107 Greenbank Road
Wilmington, DE 19808 (302) 994-1707
(Small but very busy store, the owner (Bill) is very entertaining (if you don't mind salty language) and knowledgeable. His tanks are clean and livestock (fish and corals) is healthy. Selection is generally wider than what's at Delaware's Premium Aquatics but prices on livestock are usually not marked and I often wonder how the owner can have all those prices in his head.
And lastly,
Just Fish
52 Fox Run Shopping Center
Bear, DE (302) 836-8250
(is worth a visit if you're just looking, but I think their tanks are not as well cared for and I think they're expensive, comparatively speaking. The reason they're worth visiting is they probably have the largest selection of livestock among the other two and they often have fish I've not seen elsewhere locally.)
Although not in Delaware, my favorite spot is:
That Pet Place
http://www.thatpetplace.com/
237 Centerville Road
Lancaster, PA 17603 (888) 842-8738
(Awesome Walmart sized store for all kinds of pet needs, they have the largest selection of fish, inverts, corals, and equipment). They also have the best prices around and they do mail order.