I've seen a couple threads on this but couldn't find a good citation from a rope company. I have some Bluewater 6mm nylon accessory cord from REI that is extremely stiff. It's difficult to get knots to cinch well and it's basically unusable for a prussik. I've already cut it so I doubt they'll take a return. I've seen people mention using "nylon safe" fabric softener. How can I tell if a fabric softener is nylon safe? I'd appreciate any citations available over "yer gonna die"- I would use this for anchor building so if I must buy something else to be safe I will, but I'm on a tight budget and would rather make this work.
I'll go easy on you since this is the beginner's forum...
For the same price as an entire bottle of softener (~$8), you can just buy 20 more feet of accessory cord that cinches down when you want it to. I strongly suggest you do that. Even if it's nylon safe, it's not designed to maintain the strength integrity of cord that holds the lives of you and your friends.
What Josh said. Consider whether you are in a position to properly evaluate whether any conclusions referenced in such citation have merit and are relevant to your particular facts, and whether you are properly applying the conclusions to your specific facts. My guess is that if you are asking the question, you probably aren't. Just buy some new cord. By the way, REI will most likely take back your existing cord even though its cut.
Cut your losses (literally or figuratively) and buy some other cord. While you're at it, you should consider using 7mm nylon cord for anchors. Lots of people use 6mm but IMO it's on the lighter side of acceptable.
Is your double fisherman's knot the one you're worried about? Tie it off correctly to make your loop and then tighten the crap out of it. Use pliers to pull the tails if you need to and make sure the knots butt up properly. The more you use it, the looser it will get until it breaks in.