Wait, did you take the pig back to Petco or not? The poor pig is probably either dead already or sitting in the back room without treatment.
Aquariums are not suitable housing especially if the pig already has URI.
If you have the sick baby, you shouldn't have introduced them. You have to quarantine any new pig you get unless you get her from a reputable rescue. The baby probably passed on mites and URI to yours.
The instructions for the cages are on the main page.
http://www.guineapigcages.com/cubes.htm