I had my first marine tank in 1982 (I was 11), when my dad bought me a 20-gallon setup as I was healing from a bus wreck. A few days later he filled the tank with a couple of damsels, and angelfish of some sort, and a seahorse. Needless to say, only one of the fish, a damsel, made it very far.
Then in 1991 I set up my idea of a "reef" which consisted of dead rock topped by dead coral skeletons. Hard to believe I used to prefer that look over a "natural" reef. I didn't know any better. After experimenting with a 20-gallon, I bought a 75-gallon with a wet-dry filter. I let the tank cycle and proceeded to buy the best-looking fish in the store -- a 4" GORGEOUS Passer Angel. That fish was so mean, so tough, that I could never house another fish in the tank with him. Lesson learned: go with peaceful fish if you want a community. That Angel lived until around 2001, then I tore down the tank.
Now I'm up to speed: quality skimmer, metal halides, plenty of live rock, everything I didn't have before.