I think "expert" means expert with that particular fish but maybe not all fish. For example if you have kept copperbands (more than one) for their presumed lifespan of 10 or 15 years, I would call you an expert for that fish.Again, what is it that defines an Expert reefer from a Novice reefer and a Novice reefer from a Beginner?
If you kept a couple of copperbands for a couple of years, you are not an expert but you would be a novice.
That is the same with all fish. A mandarin should live about 10 years and if you have kept a "few" of them for that period of time, you could be considered an expert. I believe you should be able to keep the fish for at least it's presumed lifespan. If it dies of anything else, it is a failure., maybe not a complete failure, but still a failure.
If your fish are not dying of old age, you could not be called an expert