I woke up this morning with a Wiggles song in my head--the one about Uncle Noah's Ark. That got me thinking about the Ark and I realized that fish would be a real problem for the Ark hypothesis. But of course those wise creationists have already thought this through!
If the whole earth were covered by water in the flood, then there would have been a mixing of fresh and salt waters. Many of today's fish species are specialized and do not survive in water of radically different saltiness to their usual habitat. So how did they survive the flood?
Note that the Bible tells us that only land-dwelling, air-breathing animals and birds were taken on the ark (Genesis 7:14-15, 21-23).
How indeed? Well there are several hypotheses.
1. The saltwater/freshwater issue could just be a post-flood phenomonon
2. Many fish are able to adapt to fresh or saltwater within their lifetimes
3. Freshwater and saltwater could have remained as separate layers of water in the flood waters
There are many simple, plausible explanations for how fresh and saltwater fish could have survived the flood. There is no reason to doubt the reality of the flood as described in the Bible.