I think this is a bacterial infection and probably a Streptococcal infection
http://www.aquaticcommunity.com/tilapia/Streptococcus.php. To positively identify this would require a lab so this is a best guess based on what your picture showed, my experience and what is known to be common in Tilapia.
This fits with the low Dissolved Oxygen conditions you've been reporting. Did you ever manage to get the DO up in this tank? Are the catfish in the same tank or a different one? How many fish in this tank and how much filtration capacity attached to it? You probably have too many fish or too little filtration, these are opportunistic pathogens that take advantage of favorable conditions to cause infection and they are probably present as part of the normal bacterial flora (but in very small quantities)(I'm not 100% certain these are normal flora but pretty sure it's the case) .
Humans also carry around several different types of Streptococci as part of their normal flora but you only occasionally hear of someone having a strep infection. Cooking will kill this organism but you have to be careful with food prep and I would recommend not eating fish showing any signs of infection like this one does or with similar lesions/blisters/ulcers in other locations. It's also a good idea to wear latex exam gloves when handling a fish like this. There is some evidence that certain types of streptococci that cause disease in fish can cause disease in humans.
Streptococcus iniae and Streptococcus agalactiae are the two main Streptococci that affect Tilapia. I think S. iniae is the more lethal to fish but least common (I'm not certain of this though). The information is probably in one of these locations which will give you a good background but not many pictures
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https://en.wikipedia.org/wiki/Streptococcus_iniaehttps://en.wikipedia.org/wiki/Streptococcus_agalactiaehttps://wwwnc.cdc.gov/eid/article/15/12/pdfs/09-0232.pdf
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