First, be careful not to over-fertilize, as the roots can get nitrogen burn and this will affect the overall health and appearance of your plants. The African Violet is actually a very difficult plant to keep alive, from what I hear, lots of people struggle with getting them to thrive. For what it’s worth, I had one a long time ago and it was so happy… I was a lackadaisical plant mommy and found by accident that the key was to pretty much neglect it (i.e. do not water a lot). As for the others.. what kind of soil are they in now? Are they still in the store-bought pots? Small, medium, or large? If they’re in small pots and you foresee having to repot them soon, I would just invest in some Miracle Gro potting soil (easy and gives you 3 months until you’ll need your plant food). Or if you get a non-fertilized potting soil, follow the directions on the container of MG plant food. It should tell you how often and how much to use. Yes, plants need feeding periodically, particularly potted plants. This is because the plant will eventually exhaust the nutrients in the soil with no natural way to replenish them (rainwater, plant matter decomposition, etc.). Lastly, what kind of plant it is makes a difference where growing conditions are concerned. Some like lots of water, others not so much. The ones that get watered a lot will lose more nitrogen than the ones getting watered less. Nutrients required are also determined by the type of plant. The best thing to do is figure out what plants you have and look up their preferred conditions.