You need level 3 of the faction you chose in order to get the next herb. Then you can make the potion to get rid of the sharks. In the meantime, the absolute best thing to do, in my humble opinion, is to not play your game for 4 or 5 days; maybe even a week. Pause your game and turn it on once a day in order to have your chief make food. I know it's hard not to play them, but it's the best way to save them. It's possible to do it by changing the clock on your computer (while paused) but it seriously messes with events and collectibles and has the potential to really mess up other things as well. For instance, if you have a nursing mother and you pause the computer and move the clock up a day so the chief can make food; then for whatever reason you need the calendar correct on your computer (my reason is my husband!) and you pause it and put it back - the nursing mother will have like 1,000 minutes or so added to her nursing time. Also, the chief will not make food the next day. To the computer your chief has already made food that day and you have to wait another 24 hours. Woe betide you if you change the clock more than one day!! No, it's best to simply pause your game for a day, unpause and have the chief make food, and then pause again until the next day. You need to have at least 200 food in order for them to work and not be worried about food. However, with so many villagers that amount will not last long. I recommend at least a week of your game paused except just long enough for the chief to do his/her thing.