You are certainly correct. Most tropical fish, I would never consider adding to my tank. Thankfully, Cory Cats and Goldfish overlap in some parameter areas, either higher or lower. I have found that a healthy, non-stressed Cory can withstand and adapt easily to suboptimal conditions.
I have a 120 gallon open top tank. The filter is a FX5 filter. All three stages are filled with biomedia. The only filter media I use are coarse and fine filter material over the bio material. I always clean my media with tank water. I am diligent about water testing and changes.
The tank was initially setup and cycled for freshwater use 3-12 years ago. Most of my Cory's have resided in the tank since then. They are wonderful bottom cleaners. I do supplement their diet with shrimp pellets. They love water changes. They will swim from top to bottom as fast as they can several times. They are fun to watch because they are always active.
I acclimate my fish over a much longer period than suggested (3 hours) and the room is always dimly lit with no tank lights on.
Right now pH is the problem for my fish. It naturally runs high which the goldfish tolerate very well, actually love. The Cory's should have a much lower pH but have adapted. I am currently working on a RO, tap water combination that will lower the pH while stabilizing it too. I tested water professionally for 10 years. PH follows alkalinity. Alkalinity is the pH buffer. Trying to use chemicals to maintain your pH causes many stressful fluctuations for the fish so I will not go this route. The RO water has a pH of 6.6 (without the resin canister), while my tap water pH is 8.2. This tells me that the RO water probably also has a low alkalinity. I need to get an alkalinity test kit to be sure. If I can find the right combination of the two, I should be able to accommodate the Corys and Goldfish by maintaining a healthier pH. It will take at least one complete tank water replacement over time before I can start very slowly adding the combination, when I find the right one. Right now I am just doing daily 10% water changes so as not to affect the parameters too drastically.
I also keep Plecos with my goldfish. I just sold a 10" Pleco to my LFS ( I know the size because he insisted on measuring before making an offer. My Pleco was spotted with good healthy green coloration. He was beautiful. When the LFS put him in the tank with the other plecostomous, they dulled in comparison). I loved him but he had to go. He was a perpetual waste factory.
I still have much to learn and was thrilled to find this group. I have investigated others and they were a laugh. The moderators and members here have a wealth of information. I have made mistakes along the way and lost a beautiful large bicolor Oranda because of it. I was very attached to her and it was a tragedy. I have learned the important lesson that you do not result to chemical or antibiotic treatment until you have tried Epsom salts for swelling and salt for many parasites and bacteria. I also learned from JLK that test strips are not reliable. She was certainly right. My test strips said everything was dandy. When she highly suggested a good liquid test kit, I abided. She was so right. Nothing about the liquid test agreed with the strips. The liquid test is accurate and should be trusted. The test strips are in the garbage
I will continue to make mistakes but thanks to this group, I think that they will be fewer