Not a good practice, at all..... Ethanol will absorb the the water and actually fall out of the gasoline making the tank issue worse....... its called phase separation - you'll be left with sub-grade 83 octane unleaded which will cause engine "ping". Your best bet is find a shop with an air pump (for safety reasons), suck out everything in the tank and dispose of it, then start over with non ethanol 91 octane.