Nah man 1/2 tsp to 9L is bugger all, i gave my little girl about 1/4 of a capful to 9L and she's fine lol. That would be about 20mLs hey!
Dunno mate, got me completely buggered. You definitely should be watering with pH'd water though in coco, i know that much. The buffering only works if pH'd water is used otherwise it takes too long to rebuffer and puts it out of whack. I also read that some people water coco like hydro and dont let it dry out, so friggen confusing lol! I'd still water once every 2 days with water for a week and see if they pick up! But dont soak the medium, just give it enough water for a drink and check on it the mext day, if it needs more just give it a touch..