They will probably survive with no food.. but you will notice a BIG difference if you feed them a little bit :-) Any hydroponics store will sell quality food. Maxibloom from GH is a 1 part food.. super easy.. just add water and feed. I have found the ph is usually a little low with it.. so you may want to get some pH up and a little testing kit as well.. pH is VERY important for healthy plants :-) If you want to take a little more time and care with it you can move up to a 2 or 3 part food system.. it requires a little more attention to detail because you need to mix the food from a few different containers in different amounts etc.. but your plants will perform better with this type of feeding! Those look like they are maybe 3 to 4 weeks into bud.. you will have at least another 4-5 weeks or longer depending on the strain :-)