If I have to venture from my normal supplier to please my customer and keep the traffic coming in my store then that is what I am going to do. I may not even make anything on the fish. But while there maybe the customer will buy supplies.It totally depends on their supplier.
I can ask my lfs for A. Stalsbergi, for example, which his supplier doesn't carry. But I can order them from tuic. The petstores will also open accounts with the suppliers that offer them the best rates. And the those suppliers don't always have everything. Sure he could order them from a speciality site for me but I'll end up paying way more because the lfs has to make a profit too.