Yes, that happens all the time. When I bought my 10-gal they told me I could keep 2-3 goldfish in there. They told me to buy a larger tank when the fish started to look 'uncomfortable'. Fortunately, I planned on getting a larger tank and wanted to just learn with the 10-gal, and it all turned out well and my goldfish only stayed in there for a short time. I learned about cycling, water changes, quarantine, disease, and medicine all very quickly.