I would think it would be but ammonia test it (get 100% ammonia and just douse the brick outside or in a bowl or something XD) and vinegar test it. If it foams up for either one, don't use it because it will alter your water chemistry. Calcium carbonate (what bubbles when vinegar makes contact) will boost your pH and kH
so just make sure it passes both tests.
Also soak it in water for a day or two after to see if it disintegrates (just to be safe)
Best of luck with your industrial style tank! - Raymond