I usually sell them to retail. But sometimes, I'll have that one shady villager come up to me wanting to take the little parasite off my hands for some nefarious plan of theirs, and then I'll see a flea on another villager 5 minutes later. lol
I used to throw them into the pond, but then one time, my game froze and crashed on me when I was about to release it. Literally thought my game had corrupted for a sec. Scary...
Usually, I try to sell it so that it won't end up in some villager's house. I made the mistake of catching one too many fish for Hans and now he has 3 fish tanks in his house....
I just release them into the river too, since that's how they're typically dealt with irl. I can't imagine them selling for much, plus I'm lazy when it comes to taking a trip to Re-Tail for one thing. One time Erik pinged me before I could get rid of one flea and he was so desperate to have it. No way was I putting that garbage in his house, dirty dirty Erik.
I sell them, I don't really want fleas in my house and if I can't put them in my house where else would I put them haha. The dialogue for when you catch a flea on them is funny and it's pretty weird on how much they tend to overreact about having a flea on them despite being animals.