The manufacturer informs:

Fast-setting cement-based glue intended for joining large stone and ceramic elements. It has no toxic compounds, making it completely safe for fish and invertebrates. How to use it: a small quantity of glue must be mixed with water until a uniform, plastic consistency is obtained and immediately start joining the elements. We recommend 'dry' gluing. If you need to glue in water, you must switch off all circulation and recirculation pumps, glue the individual elements and allow them to join for about 15 minutes. Temporary turbidity of the water is not harmful. In crowded tanks we recommend not gluing more than 2-3 elements per day as the resulting deposit may slightly increase the pH value in the aquarium.