Black background for back of tank

okay so today I put a black background one the back of my tank and also on one of the smaller sides that faces a wall. (in other words the corner) and now my fish seem unhappy -- is this a bad sign? should I remove background or will they adjust?

Penn-PLAX Double-Back Roll-A-Scene Aquarium Background – Double Sided Roll – Midnight Sea (Black) / Caribbean Blue – 32” x 50' : Pet Supplies

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