Paul Flynn, inventor of SmellyWasher, recommends these tips to avoid foul smells in front-loading washers.
Use detergent labeled "HE" for front-loading high-efficiency washers. Powdered HE detergent is best, but harder to find than liquid.
Use a fourth of the detergent amount recommended by the laundry manufacturer unless some items are badly soiled.
Use dryer sheets instead of fabric softener. Most fabric softeners are too thick for front loaders and are a primary reason for mildew buildup.
Keep the washer door ajar after use to allow drying. If your Neptune has an interior light, you might want to remove it.
Don't use powdered lemonade as a cheaper substitute. The mildew will feed off the sugar content.