I don't mind seeing a little algae or mulm, I look first for healthy, happy fish. If I see dead fish I won't purchase fish from the tank, no matter what.
There's one store where I shop that has plants in most of the tanks, and driftwood/decorations for fish that need hiding places (brownie points!). Some of these tanks are a bit cluttered looking and have a bit of mulm/algae, but the fish are well kept and look healthy.
My local Big Al's usually has only gravel in 90% of the tanks and they're kept VERY clean with daily water changes and glass cleaning. Even though the tanks are pristine, I often see dead or stressed fish and pass those tanks by.
There is another store in my neighborhood where the tanks are genuinely dirty at times, and to me that just says the owner doesn't care enough about his stock. The plants don't receive adequate light and are usually languishing in tanks of green water, fish are frequently diseased and live in bare-bottom tanks where the build up of waste is all too evident. Even the store's carpeting is grubby looking - I have purchased supplies there on occasion because it's close to home, but won't even consider buying fish.
Bottom line, it's the way the fish look, not the tanks, but it is important to me that the tanks look reasonably clean - I can't tell if the fish are healthy if I can't see them through a haze of dirt or algae

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