The (RC Polyproline) resin coat prevents water and chemicals from wetting the paper which is wood pulp based. Atop the resin is a coat of gelatin laced with silver salts. Exposure under the enlarger triggers some of the silver salts and they become developable. The developer identifies those silver salts that have been exposed and reduces them to metallic silver and a halogen (chlorine, bromine, and iodine). The metallic silver remains imbedded in the gelatin forming the black & white image. The halogen component is dissolved in the waters of the developer. Unexposed silver salts are left intact.
A water rinse or acid stop ends the developing. The fixer is a solvent for silver salts but not a solvent for metallic silver. The fixer purges the emulsion; the silver salts go into solution in waters of the fixer.
Now we wash the photo paper to rid it of residual chemicals. The resin coated paper rinses clean in less than 5 minutes in running water. If not cleansed of residual fixer, its sulfur content attracts metallic silver and it tarnishes, turning brownish. It takes time for this action. It is not too late.
Place the prints in a tray in the sink and wash in running water for 5 minutes or more with agitation. No rinsing agent is needed. If you must use something, stop the running water and add a tablespoon of table salt to water in the tray and agitate. This is an old trick that works to purge the fixer from a gelatin emulsion. It was discovered during WW2 as a way to reduce washing time from 30 minutes to 10 minutes (it works and is the basis of modern fixer clearing agents).
Adding a drop or two of peroxide will give the print archrival qualities. Rinse in running water to purge the salt and peroxide bath.