I'm not an expert on tanks or fish but it seems to me that going to a department store for equipment is something you should do only if you are an expert on what you are buying.
You may pay a bit more online or at a specialty store but the extra amount buys you the expertise and/or time of the person helping you.
If a customer asks me a question that I can't answer (which happens quite often), I can't bluff my way out of the situation because I know the people on the other computer are just as capable as I am of looking up the answer.
What I can do is research the question for them, saving them the time of having to do it themselves. That's the value I add. When someone does that for me, I am grateful for the service and the peace of mind that it provides. To me, that's worth a couple of extra bucks.