What should I do with my 600 gallon?

Sounds like an awesome tank,throw in a photo if you can.I guess the most active fish in your current group are the giraffe and Arowana...If I had that tank I would most likely maintain the stat quo or look into the longshot of going back to salt and getting a shark...say a PJ or a bamboo species.I'm always tempted to go back but I doubt if I will anytime soon.I wouldn't go brackish as I'm not into them enough to devote such a massive and awesome tank to them.The Columbian cats would provide a ton of movement but I see them growing big enough to eat the other inhabitants....Some peacock bass,namely kelberi would ad a nice splash of color to the current situation.

The water changes were just what got me out of keeping salt.I realized I was throwing away money with each change due to the salt going down the drain with the water lol.

If it was me, I would sell/trade all the big fish and set up a SA community tank. Get some large driftwood, large plants (real or fake), sandy bottom, and fill it with plecos, geos, severum, silver dollars etc. You'd have your fish movement that you are missing alright!

While it would be generally for from colorful, I've always wanted to put together an "Ancient Species" tank":
- Silver Aro
- S. American Lungfish
- Royal Clown knife (or Golden CK - I miss my old gold one)
- a few FL. Gars
- a couple of Ornate Bichirs (I really like the upper jaw species, and the ornate get the biggest)

I am going to have this one day, but you could do it now! Live my dream

It sounds like the decision is really stressing you out and stress is not good for you.. I think you should just sell the tank to me and make it easier on yourself!!! LOL.. But if you really must keep the tank I would keep the current stock and just add some nice Chicla's ( Peacock Bass ) in with them.. That will brighten up the tank and give you plenty of movement!!!

Cichla... Sorry mis-spelled that..