Thinking of why you got chewed out for trying to redirect the customer, and the notion you mention that your store knows it needs to make big changes, then maybe what both your boss and you would benefit from is a really good "sit down" together, to discuss where the boss wants to take the store in the coming months/years and how your boss views the "in-coming" notions of customers. By that I mean, some bosses believe that (A) the customer is always right and (B) if the customer wants something, just give them what they ask for, don't question them as to why they want it.
While both of those are good strategies in the retail marketplace, making for happy customers most of the time, there is room for the educational element you mentioned above (ensuring that customers are well informed) and for probing customers* about their own long-term outcomes: What do they want their tank to be? Do they see themselves getting more into fish over time, or is this just a one-tank household for a kid, etc. (all the stuff we ask each other here when someone comes along and says, "I think I want this fish - what could go wrong?")
And obviously, don't hit every customer with these questions the instant they walk in the door. I can see it now:
- Teresa: Hello, welcome to our fish store. Can I help you today?
- Customer: Thank you. Yes, I'd like to buy a 10 gallon aquarium.
- Teresa: Okay, but before I sell it to you, first I need to know - do you plan to buy a bigger aquarium later? Two, three, four years from now, are you going to want tanks all over the house? Or are you going to quit with one?
("And, cut!," says the director) LOL.
Back to my actual line of thought: Of course, I wouldn't approach your boss with, "We need to talk," but rather something more conciliatory, humble and respectful of the fact that the boss is indeed the boss.

But just perhaps, if your boss would be willing to have such a talk with you, you two can reach harmony and change a little sooner. Maybe then he would help you set boundaries for what he feels is fair game and off limits. Maybe he will appreciate the value of the advice you have to offer.
Cheers, Eric
*
Funny thing about that, as a customer I'll often butt in when I overhear other customers talking amongst themselves with confusion about these types of fish purchase questions. Since I'm not an employee, no boss can fire me. Of course the customers can choose to heed or ignore my advice, but the nice thing is that most of the sales people know me, even at the big chain stores like PetSmart, and they'll often turn to the customer and say something like, "Yeah, what he said. He knows what he's talking about!"