Your pet shop is at fault here for not telling you but no one should set up an aquarium without first knowing what the nitrogen cycle is about. Your shop person, in selling several fish to you and not being willing to take them back, has proved their canine ancestry.

A pair of guppies would have been better than the larger number, assuming that your aquarium is a 10-gallon tank. I realize sometimes we have little choice in what shops e can do business with, but a wise shop starts a person off slowly (rather than going for the quick sales). They may then do several $100 a year business for decades with some customers. Your shop's business model is plain ignorant, though it is also all too common.

Can you buy enough gallons of drinking water at your grocery to do a 30 or 40% water change? Please make sure that the water is drinking water with minerals added. Keep it at about the same temperature as the aquarium. If you have a gravel vacuum, use it to remove the quantity of water you wish to add. If the aquarium has a sturdy hood, you can then set a gallon on the hood and siphon water into the tank with a length of extra airline. That will gradually add water and not hopefully mess too much with rapid chemical changes.

Then fill those gallon jugs up with warmish tap water. (Just as we would for a drink, first let the water run a few moments to get rid of any lead residue in the faucet.) Treat the water with a conditioner designed to eliminate chlorine AND bond (for a time) any ammonia in the tap water.

Distilled or RO - reverse osmosis - water has no minerals added and therefore nothing but "pure water" in it and will shock and kill fish. In hot weather is it also dangerous for people to drink that stuff and sometime there will be a spectacular law suit over that when somebody dies because they couldn't perspire enough to deal with the summer heat. Just because it says Drinking Water doesn't mean that it always is.

There are a lot of good water conditioners. There are a number of good products - Ultimate, the original Amquel, Ammo-Lock, Stress Coat - that should do dissipate chlorine and bond ammonia. Treat the gallon water containers with the appropriate bit of water conditioner. If possible, leave them where they will be about the same temperature as the aquarium. Leave the caps OFF so that CO2, free nitrogen and the stuff being treated for can escape the water. A little O2 may also be absorbed.

In the next day or two you may again wish to vacuum more water out and replace it with the treated stuff. Replace the water in the jugs for your next partial change.

Another quick fix is to buy one of these containers, which is supposed to have the beneficial bacteria (in dry form), which will "kick-start" your aquarium's break down of ammonia and nitrites. Watch the "freshness" dates if they have one. The best dry ones are good for a year.

I haven't any personal experience with them, though I have a couple of samples here. (Wish you lived near the southern 'burgs of Chicago.) The reason is that I start new aquariums with a pair of fish, wet gravel from an established tank (which brings a lot of beneficial bacteria in) 50+% of the water from established tanks and if possible a filter, which has been already running in an established tank. Plants are often added too if there is enough light. The idea here is to import as much of the cycle as possible.

Do you know an aquarist with an established tank? Would they give you a bag of wet (lightly rinsed) gravel, which you could carefully drop in your tank, while the gravel is still wet? If not, will those rascals at your shop sell you some gravel from a tank at about the same price per pound as they sell the "clean" gravel?

Some other things to do:

Do not feed the fish very much. After stuffing too many fish into a uncycled aquarium, the next most common way of of killing aquarium fish is over feeding them. (3rd is probably not doing periodic, partial water changes.) Especially when the tank is chemically so unstable, only feed what can be cleaned up in a few seconds. They can get more some days later.

If you can get a test kit, measuring for ammonia, nitrites and nitrates, do so. Ammonia tests for aquariums have been criticized for not being detailed enough, because they measure parts per million (PPM) and actually fish may be hurt (gills burned and suffocation begun) with PPB (a certain number of parts per billion), but knowing if there is even 1 PPM or more in there is better than not knowing. We are often amazed by how toxic some aquariums' water can be!

Do you have a filter you could put fresh (rinsed to get rid of the dust) activated carbon in? Remember that activated carbon is used up in a week or less. There are other more expensive resins (Polyfilter, Ammomiasorb...) which will also draw ammonia out.

Polyfilter is also useful because it starts out an off white and gets darker as more ammonia and other stuff (including some useful stuff) is taken from the water. When it turns black, I use them to scour down tank sides while cleaning broken down tanks.

You might even use that water treatment stuff on your aquarium. Those manufactures' claims sometimes suggest that their product will remove ammonia for the water. That is true but not true. Even a non-chemist such as I can understand that they "bond" the ammonia to a substance in the treatment, but in time that bond breaks down and the ammonia is released into the water again, hopefully in a gradual manner so that the beneficial bacteria can break it down.

Does you tank have a light? Would it give 1-2-3 watts per gallon? If so, could you (without overheating the tank) get some fast growing plant such as hornwort or watersprite? If illuminated for 12-14 hours a day, that stuff will absorb some ammonia. (There is a little bit on appropriate plants in Immediate Help.)

You might Google your urban area, state or region plus Aquarium Society. See if they have someone nearby who could help you or sell you some plants and "used gravel." ;) They may even have a meeting (with mini-auction) or like the club in Champaign, IL. have an larger (300-700 item) auction coming up in two weeks.

Ironically, if your aquarium is where it gets direct sunlight, you may want to pull the window shade so that it doesn't get too much light, if it is on the tank for more than an hour or two.

A level tablespoon of aquarium salt or agricultural feed salt (NOT table salt) in the water may also retard nitrogen poisoning. When making water changes, don't just add another tablespoon of salt. If you do a 40% water change, add .4 of a tablespoon. :)

In time, your fish may be better served by not adding any sodium chloride. This is an emergency situation now.

After you have done what you can for your tank, please do that survey. Either reply here or (better yet) submit a diary.

If you are in the habit of stopping by a big box book store from time to time, look for
* The Everything Tropical Fish Book by Carlo Devito and Gregory Skokal

or

Setup and Care of Freshwater Aquariums (Animal Planet Pet Care Library) by David E. Boruchowitz.

Over a cuppa coffee or the beverage of your choice, take notes. ;)

Or order one or both of them through your public library's inter-library loan system. It is quite likely that your tax dollars have already paid for that service.

Of course you could buy one of those, but you may need the extra cash for activated carbon. ;)