Adding too many fish to a new tank too quickly causes "new tank syndrome." Because ammonia/nitrite-eating bacteria haven't had time to colonize the tank, ammonia buildup occurs. Only one way exists to fix it immediately, but several means exist to mitigate the problem, and to prevent it.
The direct cause of new tank syndrome is ammonia. Water changes are your only choice for reducing ammonia in the immediate term. Drain about 25 percent to 50 percent of the aquarium water and replace it with new, clean, dechlorinated water. You may want to invest in a test kit for ammonia. Most pet shops sell these. If you can detect ammonia in your aquarium water, you should perform a water change. Any detectable level of ammonia will harm fish.
There are ways to speed up the process of cycling an aquarium. These techniques may mitigate new tank syndrome but are better employed before you start getting your fish. The biggest way to jump-start the nitrogen cycle is to "seed" your tank with sand, gravel or decorations from an existing aquarium. Another way is to keep the pH of your aquarium above neutral and the temperature between 83 and 87 degrees Fahrenheit. Avoid suddenly changing the water conditions if you already have fish; sudden swings in pH and temperature can kill them as quickly as excessive ammonia can.
One method of preventing new tank syndrome from occurring is fishless cycling. Unfortunately, you cannot perform this process once you've added fish to a tank. In fishless cycling, you use household ammonia to cycle an aquarium without fish. In this process, you use an ammonia test kit and unscented cleaning ammonia with no surfactants. You can tell if your ammonia has surfactants by shaking the bottle. If it foams, it has surfactants. Add enough ammonia -- it's probably only a few drops -- until the water reaches 5 parts per million. Test daily, adding enough ammonia to reach 5 ppm. When it takes your tank less than 12 hours to go from 5 ppm to 0 ppm, perform a 75 percent water change and add your fish.
Silent cycling is another pre-emptive strategy for heading off new tank syndrome. In silent cycling, you plant as many aquarium plants as you can. Plants absorb ammonia and other nitrogen-containing compounds. However, this works only if you know how to adequately care for aquarium plants -- and it's trickier than it looks. Aquarium plants have their own care requirements beyond those of fish. Plants without adequate care will die, rot and add even more ammonia to the aquarium.