You made an understatement when you said that "the petstore completely misled us about how best to establish a new tank" .
Not only is the tank too small for the fish you have but these fish are not the appropriate types to use when cycling a tank. I don't to what extent you're aware of the cycling process but just in case I've listed a link below which will supply you with a brief overview.
As your tank is passing through the cycling process and hopefully nearing the end of it, the presence of nitrites is inevitable so what needs to be done is to try and not worsen the nitrite situation.
Ok as for trying to put nitrites under control.
Drastically reduce the amount of food you are feeding the fish. Feed the fish only a small pinch of food once every other day. Excess feeding and the subsequent fish waste produced are fuel for nitrite growth.
Normally I would suggest adding aquarium salt instead of sea salt to the water but as you have catfish in the tank it's best to not use any kind of salt as catfish do not react well to it so leave the salt out.
Do not change or clean the filter media as this is where the beneficial bacteria that breaks down nitrites will colonize. Once all traces of nitrites have been eliminated should you then change/clean the filter media.
Continue to do partial water changes every 3-4 days. Change between 15%-20% of the water when doing the water changes.
Try not to add too many additives to the water, i.e., bacteria, nitrite removers, etc.etc. as they will not be of any help and some might even take the tank back a few steps.
Please keep in mind that what I've recommended is not a quick cure as none exists.
What's happening in the water is normal for a fairly new tank set-up such as yours it's just a matter of trying to lessen the impact.
On a final note: be wary of advice given by pet stores/shops as many a time their number 1 priority is making money and not on a customer's healthy aquarium.
Basically problematic fish tanks and aquariums are good for pet store business.
Best wishes and please let me know if you have any questions.
Edited by TROPICAL FISH AFICIONADO on 2/2/2010 at 11:01 PM EST