I feel terrible if this is what cause my problems. I followed the humble fish hybrid tank transfer method in the forums. That’s where i got the peroxide bath info. Sounded like lots of people had great success with it, but I don’t know what else it could be. Below is the protocol.
I've done hybrid TTM per humblefish with peroxide baths tons of times, no issues. The main thing though is that they can't have external injuries as peroxide will exacerbate them. Also, you'd need to be cognizant of doing it if they aren't eating and looking thin. Lastly, it might not be recommended to do peroxide for mandarins due to their slime coat. I believe it is different for some fish, especially scaleless fish, on whether peroxide baths are recommended.
Lastly, TTM would require you to be hatching and putting in live baby brine shrimp multiple times a day to sustain a mandarin, let alone two going through TTM.