I would stay away from chemicals, as they often create more problems than they solve.
Have you checked for ammonia and nitrites, as well? If both are at zero, then your biological filter is probably just doing its job. How high exactly does your nitrate get? 20ppm and under is ideal. You have to start worrying if it gets above 40ppm. Water changes will always lower your nitrates, unless you've got ammonia/nitrite/nitrates coming out of your tap.
If you're overfeeding your fish, all that decaying fish food could be building up your nitrates. Really getting down into the gravel with that vac can be a big help, even if it means rolling up your sleeves and getting a ladder. Could there be any fish that has died somewhere in the tank that you aren't seeing? A dead animal in your tank is going to be a constant source of ammonia which, as long as you've got the biological filtration in place, build up your nitrates.
After you've checked for all of this stuff and there's still a problem, you might want to just do water changes more often or consider getting some fast-growing live plants, which turn nitrates into biomass. It would be tough picking a fast-grower that your silver dollars wouldn't just devour, but hornwort might work.