Welcome to the MFK!
Congratulations on keeping your pacu for 27 years. It is quite a remarkable achievement!
Sounds like you know a lot but need a bit of guidance.
Most usually an illness results from stress. Must ID and eliminate the source of stress. If the illness is in the acute phase, must treat too but again should have at least a good guess at diagnosis, not treat blindly. Mela and Pima are mild antiseptics while, if the ich is the correct diagnosis, it is an external parasite. No logic here.
SP: cloudy water usually means trouble with the nitrifying bacteria,
TBTB: it does often when the system is cycling, yes, but in your case it could be simply the medication
SP: and worried about oxygen
TBTB: yes, mela and pima deplete oxygen, usually when people use these, they add aeration / stirring. half the peers don't believe mela and pima do anything, half thinks it does something.
SP: with her swollen lips.
TBTB: i dont see a link of the lips with oxygen
SP: Her spots are gone, but seems more agitated today.
TBTB: stress
SP: I can continue with both Pima and Melafix,
TBTB: diagnosis or a good guess first
SP: I know water quality is where this begins
TBTB: YES! good job.
SP: It's so frustrating trying to figure it out
TBTB: it doesn't have to be
SP: Temp, ph, nitrifying bacteria, ammonia, nitrites, nitrites, filtration, particulates, media, oxygen, sunlight, water hardness,
TBTB: temp is trivial, pH not important except MUST be stable, NH3 NO2 NO3 is basics, particulates are totally irrelevant except for your viewing pleasure, oxygen is critical but trivial, lighting is of 10th importance, water hardness is trivial
SP: I'll see if I can figure out photos
TBTB: button in the left bottom corner of the reply window says attach files
SP: Would you suggest every two weeks for water changes, or test more for an as needed guide?
TBTB: I'd suggest 50%+ daily until the fish is healthy and 50% weekly or more from there
SP: I'm always worried about losing nitrifying bacteria with too much siphoning, too many water changes
TBTB: these do not affect nitrifying bacteria, they live on surfaces, not in the water column, they live ON you filter media
SP: or filter cleans. I
TBTB: that is a valid worry, you dont want to clean out filter too often, nor too infrequent, now having two canister filters it should be a charm - do one at a time, stagger; most important thing with canister filters is that water entering them MUST be as aerated as possible in your tank, best if drawn from the surface, not from the bottom of a largely stagnant tank or from a dead spot, that'd be terrible
SP: I've added nitrifying bacteria a handful of times after ammonia spiked, but it didn't seem to help. Actually the reverse a few times. Lots of factors to that maybe? Types of bacteria, water changes etc.
TBTB: NBs need long time to settle and start growing, what you added were seeds, it takes time to grow and the right conditions, one of the most overlooked is the oxygen, NBs need oxygen, the more, the better they will work for you, which is why wet-dry, biowheel, shower type filters are in general 10x more efficient per volume of media vs canisters, HOB, barrel, bucket filters etc.
SP: the tank was too small. Equivalent to us standing in a closet!
TBTB: good thinking
SP: Everywhere that I've read online, following inch of fish to gallons of water,
TBTB: This is for guppies and bettas
SP: says 500 gallons is sufficient. I have a friend who builds ponds, and has many carp outside, then inside. He's offered to help me build a bigger tank. Do you have thoughts on how many gallons would make her happy, but not too many, if that makes sense?
TBTB: for large fish it is 1 cubic inch per gallon, so say you pacu is 36" long 18" tall and crudely on average 3" wide = 1944 cubic inches! = need 2000 gal of water. and this is minimum. More is better. I may be off by a factor of 2, maybe she is only 1000 cubic inches, it is still 2x more than what you have now. again minimum.. so for comfort double the minimum, come back to the same 2000 gal that's roughly a 12ft x 3ft round above ground pool, cheap too. even vinyl liner will suffice if no folds , no creases, otherwise she will bite them up and make million holes.
HTH.