Answer: Aphids feed on foliage so if the pests are in the soil, you're probably dealing with fungus gnats. These look like tiny flies. They feed and breed in decaying organic matter so are naturally attracted to moist potting soil. You can plant your new plants outdoors and the fungus gnats will go away, especially if you use a dry mulch over the soil after transplanting. A dry barrier will keep fungus gnats from breeding and feeding in the potting soil.
Best wishes with your new plants!
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