I think many people tend to underestimate the bioload or potential size of some fish like oscars or plecos. I'm also not a fan of feeder fish. First, they are carriers of disease and can pass it on to other fish. Second, people may buy these thinking the fish will have a better life or can be cured. That is rarely the case. If people want a nice starter fish great, I'd say go with tetras or danios but don't buy a fish without knowing much about it. This is bad for the fish and the tank.
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Rule number one: Always research a fish before you buy it.
Rule number two: Always cycle your tank.