Do you think it’s safe to put flagstone in a goldfish tank?

Flag stone is fine and should not change your PH – if you do want to be sure though, pour vinegar on it – if it fizzes, this means it contains calcium carbonite and will raise the PH.

If the stone could fit in the tank, the bottom glass can hold it provided you don’t actually drop the stone and cause it to shatter (trust me, I’ve used TONS of rocks in some of my tanks) – but you don’t want to be stressing the sides of the tank by leaning too much of that weight against the side of the tank.

Sounds like an awfully big rock for a tank that needs as much water volume as you can get, to tell you the truth. It’s just not worth it if your goldfish are left with only 10 gallons of water.

However, it is safe to use otherwise and all you need to do is wash it in hot water. If you really want to play it safe you can bake the rock in the oven for half an hour to ensure anything potentially on it is killed – just make sure you cool it fully before using it. I’ve kept tons of rock-scaped setups for my rock dwelling cichlids and all I’ve ever done is rinse them in hot water.