Second time round

Not recommended but this is rec.gardens. Most seed starter i.e.perlite, vermiculite, peat, sand,etc is somewhat inert. The seed does not usually need any nutrients to get it going until the true leaves come into play. Too much nutrient(s) at this stage can harm them. Again, previously owned seed mix is best used as filler to bulk up your soils, BUT if you are going to try to reuse, you might consider some sort of heat as a first option to knock back any undesirable bad characters you might not want. Google for soil sterilization methods. Don't worry Nature will start her homeostasis almost immediately if you leave her alone. Then just worry about fertilization when and if the seeds germinate.