Beyond snails and their eggs there are a host of others, including parasitic leeches, and other insect larvae, not to mention bacteria , viruses, and dormant protozoa like ick. I (similar to RD) also dip in either Potassium permanganate, or an alum solution.
I was given some emergent plants, and lacadazically added them to a sump without a sufficient dip, the, sump tank ended up a grow out for fry, but I soon noticed the fry population diminishing, and found this.


it had grown quite large feeding on 1 inch beanie fry, but evaded detection for quite some time.