What were your biggest purchase mistakes?

When I started in the hobby I had this faulse perception that the Inexpensive stuff would preform what I needed it to do.

I bought a cheap coralife light, turbofloater skimmer and cheap powerheads, pumps etc. only to Upgrade them with better equipment. What I should have done is buy good used equipment.

buying a new pc light was dumb when I could have bought a used halide system for the same price. Same for the skimmer.