My cat just likes to drink from my tank, he's not the slightest bit interested in the fish. Glass is pretty hard, about a 6-7 on Moh's scale of hardness, and a cats claws aren't anywhere near hard enough to scratch it; probably not that much harder than finger nails, around 2.5-3 on Moh's scale. I'd be more worried about my soft furnishings and carpets
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The only bugbear I have is that the furry little bar steward goes in to the garden takes dump buries it in the mud and then washes his paws in my tank. Then he tries to dry them by flicking the water away. Consequently, the walls adjacent to the tank get covered in water and mud and probably $h*t as well, and then it gets trailed throughout the house
I keep my study door closed (that's where the tank is) to keep him out, but someone usually goes in there to pilfer my stationary and inevitably leaves the door open; and being the consummate opportunist he's In Like Flint, and I have yet another mess to clear up...pets and kids who'd have them
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