http://pinstripesandpolkadots.com/detergentchoices.htm
There are a number of sites dedicated to the choice of detergent. I would respectfully disagree with a pp who said it doesn't matter- it really, really can- though flats and prefolds are the least likely to be affected. The problem(s) with a lot of detergents are oils, brighteners, other additives that can build up on diapers and cause issues, mainly repelling and stink and rashes. None of which you want! There are different kinds of stink, they can smell poopy, swampy, ammonia-y, from using too much detergent, using not enough, not rinsing enough, the wrong additives, the wrong detergent in hard water etc. etc. You might be fine with anything, but you might encounter problems. You should find out what kind of water you have (hard, soft, mediumish) and then look at some detergent lists and go from there. Easiest is to see if yours is on a "safe" list and figure out a recommended amt and start from there :)
You also might want to start with a light bleach of any used diapers (it's the only thing that is guaranteed to kill yeast on them- which is a highly transmissible and awful to get rid of rash). Bleaching most diapers is not recommended but it won't hurt if it happens once or twice. Just follow with a lot of rinses. It will also help with sort of stripping the diapers of any other sediment from old detergents or anythign that could start you off with issues.
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