Yes, it is the water. You need to get these fish a filter and cycle their tank, ASAP. This will take some explaining:
Fish produce ammonia. They do it all the time, it's a waste product from their metabolism. Fish poo and pee also breaks down into ammonia. Ammonia (NH3) is extremely toxic to fish. Even in fractions of one part per million (0.25ppm) it starts to become dangerous to fish. The only safe amount of ammonia in a tank is dead zero.
In a tank with a 'cycled' (mature) filter, there are bacteria that 'eat' the ammonia and turn it into nitrite. Then a second species of bacteria turns nitrite into nitrate. Nitrite is toxic, nitrate is not.
Pumping air into the water will help to oxygenate it a bit, but it will not remove the ammonia, which is going to kill your fish if you don't get rid of it, real quick. The best thing you can do for them at the moment is to change the water. Change all of it once or twice a day. Don't change it all at once, take 50-60% twice a day, more often if possible. The more water changes you do, the less the ammonia will rise.
There is nothing wrong with keeping fish in plastic containers. I'm 14 and I can't afford to go and buy glass tanks because I am absolutely overrun with baby platies and guppies, and if I bought enough glass tanks for all of them I'd be looking at close to $1000. I keep mine in large plastic tubs, about 50-70 litres or 15 gallons. The tubs are all filtered. You don't need anything fancy, and since you already have an air pump you can buy a corner filter that is driven by an airlift. It doesn't matter how you filter the container, as long as you get a filter. How big is the container?
Also, getting a filter will not instantly solve your problems. You need to cycle it before it will remove ammonia from the water. Cycling is the process of establishing a bacteria colony in the filter to remove the ammonia that's killing your fish. To cycle a tank you need a source of bacteria and a source of ammonia. You already have a source of ammonia - several fish. What you need now is the bacteria. The best option is to use mature filter media - a sponge, bio balls or bio rings that have been in a mature filter will be covered with beneficial bacteria. If you put some in your filter, you will cycle the tank very fast. At the top of the 'New To The Hobby' board is a list of members who will donate mature media to new fishkeepers to help them cycle their tanks. You could also ask the shop where you bought the fish for some.
The other option is to add a bacteria-in-a-bottle product. Most of them don't work. These bacteria cannot survive for months at a time on a cold, dusty shelf with limited oxygen and no food source. The only ones that appear to have an effect are Bactinettes (UK) Bio-spira (USA) and Aquarium Science Eco-start (Australia). Bactinettes and Biospira are refrigerated, and Eco-start is freeze dried. This keeps the bacteria alive.
It would also be a good idea to have some water tests so you can tell us what your ammonia, nitrite and nitrate levels are while the tank is cycling - this will help us a lot to work out where it is at and when it is cycled. They can be quite expensive though. You should get one that uses bottles of liquid reagents. The dip in test strips are not accurate.
To sum up:
- Do LOTS of water changes. Every time you get the chance do some more water changes.
- Get a filter ASAP.
- Cycle the filter.
Keep us updated and good luck!
