That is very obviously a bacterial infection, but not columnaris. It would be impossible to find out exactly what strain of bacteria without getting a slide under a microscope.
I hate to say this, but your husband is way off based on what you put here in this thread. A high nitrate level will not "balance itself out" unless you are set up for anaerobic (denitrifying) bacteria to do the job for you, such is found in a refugium situation. Aerobic bacteria (nitrifying bacteria) will break down ammonia to nitrite, and nitrite to nitrate. In the average freshwater aquarium, the only way to effectively remove nitrate is through water changes and gravel vacs, and the use of filter medias.
How often are you feeding the fish? How much each time and what foods? Its always a good idea to try to find the source of the problem rather than just treating the symptoms. Heavy feeding is one of the most frequent causes of high nitrates, along with not enough water changes and overstocked tanks.
I'm also wondering , how big are the parrot fish? Parrotfish are known to be dirty fish due to their size, growth spurts, and messy feeding habits and high waste outputs. Angelfish also tend to have quite high waste output.
A simple solution for you would be to use something such as PuraPad in the filter alongside of whatever other media you are using. PURA Filtration Pad
Your best bet for treating this infection effectively would be to move the loaches to quarantine and treat the main tank with triple sulfa. Any of the medications safe for the other fish are not going to be safe for the loaches, and it sounds as if some of your other fish may already be infected, if not, they have all been exposed to the bacteria and should be included in the treatment with the gourami.
When using triple sulfa please take precautions to protect yourself. Many people are allergic to this stuff. It is a powder, so you want to be careful not to inhale any of the dust, and you want to wear latex gloves and avoid direct skin contact... then wash well with soap and water when you're done handling it. Once in the water you may or may not have a reaction to it when handling the water during water changes, so if you can wear protective gloves this would also be helpful. Standard surgical gloves can be used when handling the powder, but I would not use those while working in the tank. The heavy duty gloves you can purchase for washing dishes would be safer when dealing with aquarium water and its animals.
Follow the medication directions exactly and be careful not to overdose, remove carbon from the filter during treatment.
Hope this helps and your fish have a speedy recovery.