Should they be made of cement or can they be made of gypsum as well (I have not dealt with these materials much, especially in aquariums)?
What brand (type) should I choose, maybe what substances in its composition should be avoided so that it doesn't emit anything in the tank. Of course, I have aquaforest cement, but I'll probably use it later when it's about to expire.
In general, the tactic is: molding, let it harden and leave it to soak outside the tank, there is a lot of free time, they will not be needed soon
What brand (type) should I choose, maybe what substances in its composition should be avoided so that it doesn't emit anything in the tank. Of course, I have aquaforest cement, but I'll probably use it later when it's about to expire.
In general, the tactic is: molding, let it harden and leave it to soak outside the tank, there is a lot of free time, they will not be needed soon

