There are always pathogens in the tank. For them to develop into a disease there has to be a stressor. If the fish become stressed for some reason (aggression, temp change, low water quality, poor nutrition, etc.) then whatever pathogens are present have a chance to infect. You will never have a sterile environment, and you don't want it. Whether it is ich, a bacteria, fungi, virus, etc. something will infect them. The better thing to focus on is preventing the stress.
At a glance the ingredients may look similar, but there are important differences. If you read through NLS's website he discusses fat levels and how feeding a fat level over about 5% all the time will likely lead to fatty deposits and early death. IMO
nutrition is the most under-appreciated aspect of pet care, including in fishkeeping. Although the immediate results of any food may be okay, or even very good, the real issue that we have to look at now is long term health. We have good foods that provide great short term nutrition, but for fish to truly thrive long term we have to be much pickier. Long term the exact percentages of nutrients, along with the exact micronutrients (all those ingredients you wouldn't recognize if they were laid on a table in front of you) are vital.
When fish die we don't do necropsies. In most cases, unless the death was immediately preceded by a major stressor (like very cold water accidentally used for a water change) or obvious disease (like ich, fungus, etc.) most aquarists blame it on old age or something like that. In truth I think that effectively no fish die of old age. Granted there are some likely exceptions, but effectively all fish die of something we did wrong. Using salt all the time in freshwater can damage the kidneys. Low water quality, incorrect parameters (like keeping mbunas in a pH of 6.0) will cause problems. But we never find out about these things because all we see is a fish start to go down and then die.
At this point in the hobby we have the ability to truly start focusing on long term thrivability. Ideal water quality and the absolutely best, uncompromising nutrition are the two most important things to do this.