I've always found fish surprisingly hard to draw.
I think the most useful thing I've finally learned from this is that fish can only really squirm side to side. For whatever reason, I've always tried to draw moving a bit more like snakes can.
I also did a little research into homology. In my past fish TFs I've drawn the victims fingers elongating into the fins, but apparently human finger bones are more closely related to the bones at the base of the fin, while the rays of the fin are something completely different. I'll try and see if i can depict that in a way that reads sensibly!
Fun Facts:
In the Chinese middle ages, carp were called "horses" because they were believed to "be the vehicles of hermits and to run in the sky.",
They can be trained to recognize the person feeding them, and to eat directly from someone's hand.,
In the winter, their digestive systems slow nearly to a halt, and they eat hardly anything.,
They can reportedly reached a lifespan of up to 200 years.,
Some species are able to survive for months with practically no oxygen.,
They can reach a size of 40 inches.