He's not being a jerk, you are being a "know it all." And ummmm, you don't know it all. In fact you are recieving really good advise from people with lots of experience and calling us names....yipes!
Yes you did need to start somewhere, but when the lives of others are on the line, its a good plan to know their needs BEFORE you take on the responsibility. You do need to do some research.
As for your fish store lady... casual observation. The more fish she sells you,the more $$ she makes. The worse the situation in your tank is, the more $$ she makes selling you meds, bigger tanks etc... She profits from giving you poor advise and looses $ when she gives you good advise. We don't profit whether we give you good advise or bad advise. Keep that in mind.