What you're describing are heavy ich infestations. Those need to be taken care of immediately. But if the fish have a cyst here or there? Who's going to notice that on their daily walkthrough? Maybe the fish-conscious worker.
Well, to me being a little bit infested with ich is like being sorta pregnant. They spend hours in the store and are in front of and around the tanks all day. If I am in the store for 5 minutes and I notice a flashing yellow tang, why don't they? What about the dead fish stuck to the intake that is now bleached white and fuzzy? That doesn't happen overnight. The workers have walked by that same tanke dozens of times and have missed it?
As for the time thingy, think about it. You have to restock the shelves. You have to catch fish for customers. You have to educate customers about the requirements of absolutely everything. In OP's case, you have to go out and maintain customers' tanks. Maybe a cursory glance before opening and after closing, but still. Maybe a fish doesn't want to be seen, and maybe you have way too many fish tanks. Even the best people can get caught off balance one day. The least that you could do is point out the offending fish so that the person working the store can try to remedy the problem.
Yeah, owning a business is damn hard work. A cursory glance at the most important thing they have? I should not have to point out dead fish that have been dead for days nor fish so badly diseased they are near dead.
Why leave aiptasia/bubble algae? Maybe the frag was recently created, and the aiptasia/bubble algae popped up from "nowhere" as they often do. You can't remove the pest without stressing the frag, and with any animal, stressing it after it's been wounded is NOT a good idea.
I disagree here. Removing a bubble or two from a frag plug should not stress even a newly cut frag. If it did, I would not want that frag anyway.
As for the fish/decoration idea you have, exactly who are you appealing to? Knowing hobbyists who know what they're dealing with would love your simplistic setup. Newbies who come in will be horrified by the severe utilitarianism of it. They want to know what the fish will look like with sand and rock in the tank, not a chunk of carved PVC. You can't honestly expect to have one of everything in each of the show tanks, can you? Additionally, fish that don't feed at first, will often be able to find "munchies" on live rock to sustain them until they start eating prepared food. Live rock is (in my opinion) also more comforting to the fish; most fish come from a region where rock is common, and already associate live rock with safety.
As I said in my other post, set up a couple show tanks with live rock, sand, the whole shebang. You don't have to have one of everything! Keeping healthy livestock in clean holding tanks is not going to scare off potential customers. Do you think Divers Den holds fish with sandy bottoms and live rock in thanks? LOL! They hold all sorts of fish for months.
Frag tanks: I have to respectfully disagree with you. Fish are excellent helpers in a frag tank. Halichoeres wrasse will peck off red bugs and problematic isopods and amphipods. Tangs will munch off long and short algae. Certain rabbitfish will devour bubble algae. Filefish and Copperband butterflies can be trained to eat aiptasia. Damsels can provide more water movement as well as waste for the corals to utilize as food.
If they have all those sorts of pests they are doing it wrong and I would not buy anything out of the tanks that were so infested. It is not impossible to keep pest free corals. Hard to do? Sure, but not impossible.
For time and pest problems: the OP has mentioned she will hire students, who may or may not be educated when it comes to fish. A normal (non-fish loving) student isn't going to be as completely thorough as a fishkeeper. Most likely, they aren't going to care that a fish has just died. Probably, they aren't going to care about selling aiptasia anemones. They might not even care about selling bubble algae.
I kid haha. 
