When I was a fishkeeping n00b I picked up a chinese algae eater. The first mistake was that it was a chinese algae eater. The second mistake was that I picked it up to live in a 5g tank with my betta because I - oh, here's a knee slapper - trusted pet store advice on required tank size. The third mistake was then when I realized it needed its own bigger tank, I provided it, but failed to gravel vac underneath the tank ornaments for a prolonged period of time (as in never). The final mistake was one day stirring up the sub under those ornaments, which I believe must have had a build-up of some kind of toxic gas (the STENCH that bubbled up to the surface was awful), because immediately after doing so the fish started thrashing violently and gasping for air. He suffered some kind of neurlogical damage from this incident and a result would have random twitches, thrashing fits, etc. even when being kept in pristine water conditions. He died young for a CAE and I felt just awful because absolutely everything that went wrong with that fish was 110% my own idiot fault.
So lessons to be learned? Don't buy any species without thoroughly, thoroughly researching its care. Don't trust a pet store's advice on really pretty much anything, because 9/10 times you're dealing with someone ignorant, and 10/10 times you're dealing with someone who wants to make a sale. And for god's sake, don't set up a fish tank unless you actually know what you're doing in terms of properly CLEANING it. It seems like a no-brainer, but I was young and stupid and honestly didn't think I needed to clean the gravel under the ornament because surely the fish can't poop under a solid surface. Nevermind that this is occuring in WATER, duh. *shakes head* That poor, poor fish.