Ok, first let me say that these things still bother me.
Not because of the money, but because I killed stuff. This was
in like 1987 before the internet and free information and like so many
new hobbiest, I trusted the LFS

Bought a 20L saltwater setup from my LFS. Basically tank, generic hood,
salt, net, and hangon filter. Didn't bother to buy liverock
(didn't even know it existed), instead I spent about $200 on that dead
expensive ornamental coral.

3 days later, bought a panther grouper (polkadotted grouper) that was about 4 inches long

a week after that, bought some kinda anenome (probably bubbletip) from the
same LFS. The only lighting I had was the one flourscent bulb

About a week later bought a yellow tang, not knowing anything about it's
special diet.. oh yeah, the tank came with flake food.

The anenome lasted about a week, then of course killed off the two fish.

I still have thoughts of arsen when it comes to that LFS. luckily it went out of business awhile ago.