Chances are it's not the fault of the spec tank, just the conditions the owner has created within the tank. It can be due to feeding practice, iron levels etc.
At the water-air interface of natural water bodies, material accumulates to form a surface film. Surface films are altered by sunlight, water movement etc. The films are basically microorganisms and organic matter.
These microorganisms need something to eat and the owner's practices, whatever they may be, are giving the microorganisms enough to survive and sustain their numbers.
I've had it a few times over the years, in high tech high light tanks through to breeding setups. My very anecdotal observation is it seems to be strongest during times of heavier fish feeding, like when conditioning fish for breeding with live and prepared foods. Also when dosing iron in higher amounts.