For volume of fish I don't think stocking is a problem unless that silver dollar is full grown.
My first instinct would be to say it was an inaccurate test. What were your readings before this? Also, how long has the tank been established? If the tank was cycling all the ammonia and nitrites working through the system would have hit your tank with a big slug of nitrates as the cycle ended. If the test is accurate its time to reassess your maintenance habits.
First off, one of the big goals of water changes is to keep nitrates under control. Your change schedule should be often enough and large enough that nitrates stay consistently below 40ppm. If your changes are too infrequent or small then the equilibrium point will be much higher. If nitrates are that high then you need to pick up the pace or volume of water changes, maybe both. I wouldn't advise more than a 50% water change at a time. If it takes more than that do more water changes with a smaller volume, like bi-weekly 30% changes. You're going to have to toy with it to see what it takes to keep your particular aquarium in check.
Second, nitrates are the end result of decomp in the tank. Either your fish eating the food and pooping or uneaten food decomposing. Its also possible that a fish might have died and led to a spike in nitrogenous substances. If you have no dead fish then you will want too look at how much you feed them. First off, ignore what it says on the fish food bottle. Most read "as much food as the fish eat in 2 to 3 minutes." Remember, these people have a vested interest in selling you more fish food and the best way to do that is work you through your current supply as quickly as possible. Look to feed your fish as much as they'll take in about 30 seconds. If food is falling to the floor of the aquarium feed less or feed in one or two individual pinches after another instead of one big glop. If the fish lose interest feed them a little less. Only feed twice a day.
If nitrates are really though the roof temporarily cut back to one feeding a day along with once a day 50% water changes until the nitrates are under 40ppm again.