Esstee
1. dont use additives, it's pointless and only complicates matters.
2. use only the purest distilled water you can find. Anything less will potentially react with the silver ions during processing.
3. clean, clean, clean... every component of your process needs to be cleaned and sanitized so as to avoid cross contamination.
4. do heat your water(98c and above). This will help move the oxygen out of your solution and promote the reduction of silver hydroxide into silver nanoparticles. Which will in turn, stabilize your solution.
Hope this helps.