Their is a lot to unpack there but I can start off. I work at a public aquarium and I have freshwater tanks at home.
- How are saltwater fish personalities? Is there a saltwater equivalent to the Oscar or other cichlid personality types?
Every fish is a little different groupers And snappers are a little like some of the larger South American cichlids but the things with the most personality are the stingrays and puffers. Angles are VERY territorial kinda like African cichlids but are little more willing to kill each other.
- How do filtration needs differ? Seems like every salt tank I see is drilled but is that just because hey, it’s already expensive and elaborate enough I should have a sump running?
so the best way to get all the gear you need into your filter system is a sump. protein skimmers are very useful and biological filtration is important in all aquariums but in salt getting as much bacteria to colonize as possible helps keep a tank stable. Reef people tend to be the type of fish people who like a challenge and want it exactly right so a sump gives people the freedom to customize as well.
- Are there things that freshwater keepers should adopt from saltwater and vice versa? Ways of thinking, techniques, equipment, etc?
Yes and no do your research on what species you want to keep and how to best keep it. Give the animal as much space as you can and keep the water clean. an aquarium is an aquarium at the end of the day.
What’s your overall experience with the pros and cons of each?
Pros and cons depend on the person but here are some of mine
freashwater pros "free" water changes, less expensive on average, and Can be minimal maintenance.
Freshwater cons limited fish with lots of color, less tank tech like lights and other fancy equipment, and that's about it but I'm more into fresh than salt
salt water pros lots of cool colorful fish, corals, and cool setups with neat gear. Salt water cons expensive for everything, lots to learn for new people wanting to get into reef keeping, more equipment that is required to run it.
brackish pros basically fresh with a bit of salt so you treat it like a freshwater tank, cool fish and some plants, can be fun and really interesting for a fish keeper who has had fresh and would like a little bit different fish and more of a challenge. Cons have to buy reef salt, is a little more expensive to get the fish, and salt creep will begin to happen and starts to look a little like hard water stains it is easier to get off however.
please feel free to ask questions and other people will add things I hope