Right or wrong pet stores dont exist because of the animals, they are seen as a product. They have to be sold and have to make the company a profit in order for them to stay in business.
You DID cost the company 2 sales. You could have tactfully approached the customer and possibly even sent them home with a bigger tank.
Sales isnt always about long term happiness. Haha Ive sold cars before (new) knowing its not a reliable product, but bottom line the company needs money, I need money.
If you dont like it dont work there. You did the right thing by quitting.