I'm pretty nice about interrupting, but I will make it clear, that the information is incorrect. and do my best to set both the customer and the employee in the correct direction. if needed I willl approach the manager/owner of the store to do what I can to help the situation. I dont know how many times I have been just scoping out the local stores and had the local employees direct their customers to me for questions. they know me well enough now that they dont really question my advice, which I have also warned them, isnt the best Idea. you should question everything and everyone 3,4 and 5 times over to be sure the information is accurate. I've been wrong on far more than one occasion.
I love giving the new employees a run for their money on their knowledge unfortunatly, in general, pet store employees are not hired for their knowledge but rather for their willingness to work for minimum wage. consider that when approaching a person for advice about a (potentially) multi-thousand dollar tank. and the more mistakes you make the more expensive it gets.