is there a level of chloramine that is safe for fish? i have heard of ppl adding city water straight from tap for smaller wc but i would be scared to do this myself. one of my friends lost several fish during a wc where we suspect the city had flushed the water lines near her house during construction. symptoms were immediate, as she was refilling (using prime!). she double (or triple?) dosed the prime once she realized what was happening but it was too late for some of the more sensitive ones.
one reason i mentioned wc in my previous post was due to the age (3 years?) and smaller size of ur discus which i felt may indicate water quality has not been ideal over the long term. i do not think of wc as a magical cure tho... when u said the issue improved after 3 massive wc (and use of tetracycline) it reminded me of a suspected chilodonella outbreak myself and 2 others had in our QT after ordering discus together from the same source. plus your pics and symptoms matched our fish's symptoms exactly. we all tried different treatment methods and my 100% wc appeared to (temporarily) stop the progression whereas my friends had fairly immediate deaths despite using meds. the wc didnt cure it tho, just seemed to stall it a bit and we figured this was due to decreasing the # of protozoan in the tank? it was interesting to have 3 separate QT to experiment with treatment and i think it helped us narrow down our cure. it was highly contagious as well, and even jumped into their other tanks if i recall correctly.
clout was the med that finally eradicated it in all our tanks, after trying everything else we had on hand (had to order clout from out of country) and even then we reinfected our tanks a couple times because we failed to bleach our hoses (dumb, i know!) but eventually we figured out a plan that worked. i mention this because:
1) there was improvement after ur wc+tetracycline, just incase it was partially the wc and not just the med that contributed to the halt of deaths, but issues continued
2) this improvement seems to contradict my worry u add ur water straight from tap with no prime
3) i did not mention chilodonella previously because i am always hesitant to jump to conclusions, but u are now considering it a possibility so i wanted u to know just how difficult it was for us to eradicate (my fish did not experience secondary infection tho)
4) bleach worked for our contagion, i bleached and primed every tube, tool, net, bucket etc. i prob used more bleach than was needed but i was paranoid and i rinsed really well afterwards, then let sit with prime, then rinsed again and let dry. there was still a faint bleach smell to some items but i never had issues using them later. i prob would do an extra prime rinse if it was an actual tank i was cleaning tho.
im no expert tho,i dont know whats wrong with ur fish, these r just thoughts based in experiences ive had.
u seem really dedicated to resolving this, i hope they r on the mend and pull thru OK