I found these small (10-14mm) grey stones at the local hardware. The bag says that they are suitable for ponds etc . Would they be suitable for my aquarium? The shop assistant let me tank a handful home, i washed them and soaked them in white vinegar , they did not bubble or fizz at all . I am now soaking them and will pH test the water when my hubby gets home and tells me where he put the pH test kit .