For cross-country, you can look at an average of $.80/lb. It's more in some areas like NYC, Boston, and Miami it seems as well, not sure about Philadelphia but I figure it shouldn't be too bad as there alot of freight coming out of that region. Remember to include any packaging (crates, pallets, etc) in your total weight and any surcharges such as residential, liftgate, inside delivery, etc. It's not uncommon for these surcharges to add $150-200 depending on your needs. Many carriers do not want to ship fish tanks at all though (liability I suppose) so you might have shop around based on this as well.
HTH,
James