To answer your question first: Since it is a new tank, I have to ask if the water parameter okay or changed from your old one? or have you noticed him shedding mucous?
When I had a RTC, it gets agitated when I did large scale water changes...
My one and only experience with RTC was 15 years ago...I bought one at 3 inches and put him in my 100 gallon tank. at 7 inches, he killed my plecos. At 12 inches he killed my pet cichlid, a Red Devil; then he lived alone until 24 inches when I had to trade him away because there was not enough room and I can't get a 300+ gallon tank. The whole process took about a year and half.
I am not really a tank size nazi but if you want him as your kid...he demands a bedroom(literally) of a tank...If you like your oscar, I suggest strongly get a smaller catfish...or I would say leave the oscar in the 40 gallon, it would be better for him than to share a tank with a growing RTC.
When I had a RTC, it gets agitated when I did large scale water changes...
My one and only experience with RTC was 15 years ago...I bought one at 3 inches and put him in my 100 gallon tank. at 7 inches, he killed my plecos. At 12 inches he killed my pet cichlid, a Red Devil; then he lived alone until 24 inches when I had to trade him away because there was not enough room and I can't get a 300+ gallon tank. The whole process took about a year and half.
I am not really a tank size nazi but if you want him as your kid...he demands a bedroom(literally) of a tank...If you like your oscar, I suggest strongly get a smaller catfish...or I would say leave the oscar in the 40 gallon, it would be better for him than to share a tank with a growing RTC.