Customer approaches me while I'm engaged in draining a display tank to replace the substrate.
Me: Hi there. How are you today?
Customer: Ok, I guess -- have a little ick in my tank, though. Thought I'd do a few water changes to try to get rid of it.
Me: (Thinking this a not very aggressive response to ick, but whatever). Yes, I guess water changes can help ick.
Customer: What are you doing, anyway?
Me: I'm replacing the substrate in this tank.
Customer: The what?
Me: Substrate. Y'know -- sand, gravel, the stuff on the bottom.
Customer: Oh. Why?
Me: Well, after a few years the grains get coated with organics, biofilm, and phosphorus compounds, and it loses its capacity to buffer the pH.
Customer: Maybe I should do that in my tank.
Me: Maybe. How long has the system been running?
Customer: Oh, about 3 or 4 years.
Me: Yes, then, but don't change all the substrate at once -- that's really a shock to the system itself, as well as to the livestock.
Customer: The what?
Me: Livestock. Y'know, fish and invertebrates.
Customer: Oh, I don't have any of that in my tank.
Me: (recalling the earlier comment about ick, and wondering if I really missed something) Ummm...you said there was ick in the tank.
Customer: Yeah. Its all over the rocks.
Me: (A bit perplexed) Ah...you can actually see it on the rocks? Wow.
Customer: Yeah, its everywhere. Big slimy sheets of it.
Me: (Slowly getting the picture) Wait -- is it purplish in color, and easy to brush off the rocks, really slimy?
Customer: Yep.
Me: That's not ick -- that's cyanobacteria.
(The rest of the story involves said customer purchasing 40 lbs. of substrate after expecting me to sell him 2 10 lb. bags at the price of the 20 lb. bags that were out of stock. He returns in 20 minutes to exchange the substrate for a different, more expensive type and demands that I exchange it even-up. He threw a big tantrum at the counter, and I now wish that I had belittled him for being 4 years into the hobby and not having so much as a clue. I also wish I had asked what happened to all his livestock over the course of 4 years. Well, maybe I'd rather not know.)