haha we'll let's see
- get rid of all the little fish. will only keep the 3 discus, roselines, and the 2 rainbows. no more new plants or fish!!!
Are your denisoni wild caught? I've had the WORST luck with my wild caught ones... gonna go down to petsmart and just buy the lame captive bred ones and quarantine them. I've like 5/6 of my wild caught. 
- I'm not gonna worry about ph, these guys are captive bred, so they should be used to different conditions. LFS didn't do anything special to the water
I am seriously moving to the captive bred camp. Wild caught fish are a pain.
- i think they may be ok in that tank for a good while, no plans to upgrade tank sizes anytime soon. if i ever do, it will be a 75-120g
The 57 gallon oceanic right? It's got a good footprint, should be fine for a while.
- breeding, not really gonna try persay. if it happens, win. if not, no big deal
- water changes, i'm already doing 50% a week for EI dosing, so that will remain the same
- ph swing, not sure. don't really check the ph throughout the day. but generally speaking, co2-induced ph swings don't affect the fish since the TDS of the water isn't changing