I really just want to know whats out there for choices. Not really trying to focus on just a 20 gallon tank. but its what I have currently. I really just wanted to know what kind of fish out there that people would be willing to spend some bucks on. For any tank size.
Flowerhorns, arrowana, koi, and goldfish are the top 4 most expensive FW fish I know of. Monster fish can sell for a good penny as well. However if you're in it for the money these fish are very costly to breed. The catfish I collect that are "expensive" are impossible to breed/exceedingly difficult. I personally only spend a lot on a fish if I think its behavior or physiology or native habitat is interesting to me. Pick what you want based on YOUR OWN preferences, not how much it costs. The price of fish is normally based on rarity more than anything else.