I used ~200mg Ascorbic acid to 500ml of water in my GH booster, Fe Gluconate and CSM mixtures. Does okay there. I just took a 250mg vitamin C pill and ground it up for the acid. Works excellently. Refrigeration is also a good option btw. Most fungus really hate acidity. Around 4 or less pH should stop it.
I dunno if this is bad but i keep mine in the fridge. No mold after 3 weeks.