Humans have only been living in sterile environments for the last 100 years. Before that, we basically pooped in the streets and ate filthy, half spoiled food. People have been around living like that for millions of years and we had no prizapro. The reason my fish are seemingly immune to everything IMO is the gut bacteria in the worms and clams I feed almost every day. We will one day change our ideas on this hobby and embrace some types bacteria, viruses and parasites as valuable life forms that help us rather than kill us. Without our gut bacteria we would die.. It is such a simple concept and fish are not at all delicate. There is no need for them to have to be taught to eat and to make feeders for fish such as copperbands etc. Almost all fish should ravish food and in a tank with no predators they should not be shy.
I always rinse my fish food in tap water and if it falls in the sink or the floor, I scoop it up and dump it in the tank. I feed live worms with the dirt. My fish won't get sick, it just can't happen. When I move in a few weeks, that's another story.