Gluing rocks and decoration with silicone
I want to glue lava pieces and other decoration together with silicone for my aquarium to make them more stable.
lava is not really the best choice for an aquarium: The material is rough, has sharp edges and bears high risk of injuries for fish, e.g. in case they are frightened and crash into the rocks panically. Smoother rocks are better suited.
What kind of silicone do I have to use for this purpose?
You may use only silicone in aquariums that is labeled aquarium silicone, i.e. the suitability for aquariums must be expressly stated on the tube or cartridge. Construction or household silicone may contain fungicides that can lead to terrible consequences in an aquarium, as many of these substances are toxic for fish, invertebrates and micro organisms. You may sometimes read that silicones that cure by releasing acetic acid are automatically suitable for aquariums. This is simply wrong, because the above mentioned construction and household silicones as well usually release acetic acid when curing!
How long does it have to harden before I can put it into the aquarium? Can you give me some advice on that?
The hardening rate is about 1 mm/day; silicones also cure under water - water is even required for curing, therefore it is recommended to place a small bowl with water or a wet cloth inside the tank when gluing in an empty aquarium. You should properly aerate the room in which the silicone is curing: Acetic acid is released during curing, which you can smell quite distinctly, and which may also irritate mucous membranes in higher concentrations.
However, it is virtually impossible to glue decoration outside the tank and then transfer this construction into the aquarium without tearing it apart. Silicones are not the best choice for porous rock anyway, it does not stick too well on it. Mortar is better suited in such cases - but you can also not use it for gluing constructions outside and then put them in in one piece. Mortar gluings must be watered carefully (possibly several weeks!), until the pH does not rise anymore.
Dr. Bodo Schnell