In short
The varied kinds of fish in the net show the preaching of the gospel. The kingdom is the net, not the fish, since a net catches many things besides good, edible fish. As already heard in Matthew 13:41, the earthly form of the kingdom includes elements that must be purged out.
In the parable on the net the different kinds of fish gathered in the net represent the preaching of the gospel. The kingdom of heaven must be compared to the net itself, rather than the fish. A net, after all, drags along all sorts of things—not only good, edible fish. As we already heard in Matthew 13:41, within the earthly manifestation of the kingdom, there will also be elements that cannot remain in it and must be purged out.1
47 “Again, the kingdom of heaven is like a net that was thrown into the sea and gathered fish of every kind.