I'd say good bye to the Oscar for sure them, they need a 125 minimum. Same for the plecos. RDs HATE to have tank mates when full grown. Dempseys are large cichlids as well, reaching 9"+. The African don't usually mix well with south Americans. Your water is super cloudy for a reason. I'm not trying to be a jerk, just pointing out that you're gonna have A LOT of issues sooner than later, even if you do split them p between two 75s...