My first tank was a 90 gallon. The guy at the lfs set it up for me with everything I needed.
It had an undergravel filter, no sump, no protien skimmer. The decore was dead coral skeletons.
Had a list of fish that I liked so they all just went into the tank after a short cycle.
No consideration for compatability, just looked cool. After some considerable money wasted and no success I found a decent store and learned how to do things the right way.
Just lived and learned things the hard way. I am still amazed at the experts advice I got from a large, successful fish store. They obviously didn't get my business any more.