What did the do to the Fish?

I suspect you mean the new fish mechanic. It is completely different than before.

Depending on the level type they are actually quite beneficial or utterly awful.

1. Square makes one and they also appear randomly like other special candies.
2. Two fish make three, they had a baby… lol. One could say the mother fish had a jelly belly…
3. Order is jelly > frosting more than 1 layer > licorice >= frosting 1 layer > special candies unless the candy would affect the requirements. This is in general as there do seem to be some variations like if a Gummi Dragon is in a licorice cage and there are no Gummi Dragons not in cages/above the cage.
4. Special Candy + Fish = 1 special fish.
5. They are smart. They will clear directly under Gummi Dragons.
6. When combined with special candies they are usually smart… ish.
7. I think they act a little like an auto hint, make the best move possible in general.
8. They are terrible if you need tons of jellyfish in general for a level.
9. They are making updates to lower levels to work with the new fish mechanic. Level 103 in particular now has no fish on the sides and striped generators.
10. Color bomb and fish is really strong now.

Further analysis

1. They are great on levels with no jelly where the old fish were useless or non existent. Level 154 is much easier as example.
2. They are good on where there are corner moves and such that really sucked before.
3. They are terrible on levels that require many fish or multiple special candy boosted fish.
4. King has upped and lowered the number of turns for some levels. The ones they lowered 100, 114, 140 are way too tight. 114 the last dragon drops on the last turn… did literally no one test that?