Growth on saddled bichir. (PIC INSIDE)

looks like an ulcer mate,
put him in his own tank and try a salt bath,if that doesnt work try melafix.

Hope Ive helped.

I have seen similar types of irrritations on my various bichirs but with regular water changes and keeping the nitrates low (under 5ppm in a non-planted tank) you can usually get away with lower pH. They seem to do best in warmer waters, my most active and happiest pair live in a tank that hits as much as 36C. You might just be suffering from a bit of stress from the changes, so as long as the sore shrinks or loses the redness you are probably just fine.

I would take a look at the nitrates in conjunction with the acidic pH. Maybe try to do more or larger water changes to get the nitrate down as low as you can go. Also, be sure to feed them a good meat-rich diet. Mine get raw chicken bits, raw fish and shrimp and they love Hikari Sinking Carnivore Pellets (mine eat them out of my hand usually)

Your bichir is beautiful by the way...

Aquarium salt and melafix bath.