*sighs* If some clueless customer comes into the store and ask for something totally ridiculous, try to persuade him to see it your way. If they're stubborn, suck it up and give him what he wants. They'll probably failed and come back, either complaining, learn from his mistakes, or neither, or against the odds, nothing will go wrong. Somethings may never be as perfect as you may wish, but remember that you job isn't to yell in their face, as much most would want to, but to educate your customers and sell. If they ignore your advice, there's nothing you can do. Even if you refuse to sell the fish/tank/products/etc., the chances are they won't change their mind if they aren't willing to compromise. It's their problem, not yours.