Decorate tree and purchase new fish

Tree up, watered, and decorated. Bought a new molly at Pat's Pets. Our prior one died. The guy at Pet Smart would not sell me a fish because my ten gallon tank is the right size for just my cat fish. He doesn't care that my cat fish is sociable because it is labeled as "slightly aggressive". Since I had an early catfish die of anxiety when left alone too long, I reject his assessment.  I walked out of that store and won't shop there again.