Its not a huge surprise; the bacteria that work on nitrites take longer to get up to speed then the ones that chow down on ammonia.
Keep up the water changes, but you can also add some salt. Salt is THE treatment for nitrite poisoning; the chloride ions keep the nitrite from blocking the oxygen/CO2
exchange in the gills. Doesn't take much; a teaspoon or so should do it for your tank, and your guys can certainly handle it! You may even have salt in the tank already, as mollys really appreciate it. If so, thats probably whats keeping the higher nitrite levels from being a problem *grin*
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