Most of the Pittsburgh plant geeks hang out at the "other" forum - one is a moderator there.
I haven't checked out Atlantis yet, but was thinking about it one of these days. If I check out a Chiefs hockey game again I'll try to find Gittler's. I live in Greensburg so going into the city is a pain as well, but usually it's for aquarium-related activities
Another place in Monroeville is Petland, near the Starbucks. Not that great, but if you're looking for some of the schooling fish they usually have some stock on sale (neons, black neons, etc). Elmer's stock is disappointing, don't know if they have very healthy stuff alot of times. If I ever buy fish it's from AoA. They actually sell decent plants too - the manager is a plant geek. It's cool to talk shop with Tim (manager) about plants or Eric (owner) about fish.
You should make a trip to AoA next Sunday, May7 at 5:30....it's the next PAPAS (Pittsburgh Area Planted Aquarium Society) meeting.
Lot's of plants for auction - usually 1 or 2 bucks for stuff, even rare stuff as well. Oddball also stocks some cooler planted-tank related stuff as well, like Nerite snails, Cherry shrimp, sometimes some other shrimp as well (Bumblebee, green). It's about 10 minutes from the Parkway, just south of the Southside.