Jade lungfish are used in 2 or 3 dailies as well as the starting quests for cooking, so they sell well and farmers are on the jade forest waterways constantly now.
The problem with selling fish is they are really, REALLY easy to bot. Which means supply will always outstrip demand. Thankfully there's unlimited demand due to the Ironpaw Token system.
That said, Golden Carp have been my most reliable seller, but as others have stated the prices are getting lower and lower as more people finish their leveling and work on supplying the market. Prices have dropped in the past week from 200+ a stack to 75 or less on my server.
On my low-mid pop server, fish are a huge money maker. I can fish up a few stacks and flip them for several hundred gold each. Fishing out of pools or not. Salmon, Shrimp, and Carp are good. It's the best fishing's been since this game came out, in my opinion. Everyone does cooking and tiller stuff lately, I'm happy.