hello99,
If they enter the fish in a krill, we have to deal with the issue of why they don’t obtain the status of the krill (as part of its flesh) and should be assur.
We can find a way around this (such as postulating that they never exist in krill at a visible size, and that dogim t’meim never swallow worms and have them reach visible size in them), but this seems far-fetched. It is also unclear why, if we find them in the large fish (i.e salmon) at a visible size, we don’t assume that they were swallowed directly as a free swimming creature. None of this is impossible, but I fail to see why this would seem a more compelling argument than that of the gedolim.