You do realize you're talking about a fish that gets 4-5 feet long and close to 100 lbs full grown right. All those fish will be eaten eventually. I've seen rtcs grow an inch a month. It's unbelievable. No offense but why do you own this fish? I hope you have a 1000 gallon plus aquarium ready for it
20 Gallon SPS reef
29 Gallon FW "fat catfish" tank
20 Gallon Long Dart Frogs "Zig" and "Zag"
I'd add nothing, you're overstocked as it is. A 75 is on the small size for a pair of dempsey as it is, let alone adding a 7" eating machine to the mix, because RTC never stop eating, and are messy as you should already know.