I was going to leave this alone, but I can't lol!
Respectfully, none of the fish you've mentioned - goldfish, whatever you found in your local creek, and large mouth bass - are difficult fish to keep. But, there's definitely plenty of challenging freshwater fish and systems to keep.
There's A LOT to talk about with freshwater, and the proof of that is that freshwater forums do exists. My main purpose for starting this thread was to question the "Reef2Reef Fresh" selection in my account settings. And, that furthermore I would personally enjoy a freshwater subforum if they were considering one. Seems like a lot of other R2R members feel the same way.
Off the top of my head, I'd say things worth discussing in freshwater would included:
- Planted tanks and plant care (including CO2 injection, as you mentioned)
- Shrimp tanks, shrimp breeding (<<the "hottest" trend in freshwater, with aspects rivaling reef tank care)
- Other freshwater inverts (Thai Micro Crabs, "Lobsters", snails, etc.)
- Freshwater aquascaping techniques and DIY rockwalls etc.
- Tanks that look like miniature forests or mountain scapes
- Fish breeding and fry rearing
- Predator/Monster fish; Arowanas, Gars, Big Cats, etc.
- Brackish systems
- Discus tanks
- Freshwater stingrays
- All the Cichlids have a huge following
- Goldfish and Koi deserve a spot
- Conservation
- Just sharing and enjoying each other's tanks on a forum I really like :)
While I generally agree that a "typical" marine tank is more complicated than a "typical" freshwater tank, I would say there's almost an unlimited amount of topics for freshwater discussion. Some people take freshwater very seriously, and would happily debate the very finer points of the hobby. Some people have no interest in going saltwater (gasp!).