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Silicone in the Aquarium Hobby
by Frank Mueller

Silicone-sealed glass tanks have been the popular choice of aquarists for tanks up to about 240G since they replaced metal framed aquariums in the 1960s. Due to the higher cost, acrylic can only compete seriously for larger tanks or custom shapes. Given this popularity of silicone in the construction of aquariums, one might assume that it would be common knowledge what type of silicone to use for aquarium purposes. Yet surprisingly, people looking to buy silicone for aquarium projects - be it the resealing of a leaking tank or the installation of a DIY background - find themselves confronted with so much information and misinformation on the subject on the Internet, that it is almost impossible to separate one from the other. We have reviewed this information for members of, and find there is a simple take-home message:

All pure silicone is aquarium safe once cured.
Silicone with antibacterial and fungicidal additives might put fish at risk.

So far so good, but as so often, the devil is in the detail, which we will discuss in the remainder of this article.

Aquarium Safe - pros and cons of buying silicone with this label.

For those looking for absolute peace of mind buying an aquarium sealant that is labeled as 'Aquarium Safe' by the manufacturer might seem like a no-brainer. At the time of writing this article (Jan 2013) dedicated aquarium sealants are available in the USA under at least four brand names:

  • ASI Aquarium Silicone Sealant
  • All Glass Aquarium AAG65010 Silicone Sealant
  • DAP Household/Aquarium Adhesive Sealant
  • Marineland

In addition, aquarium sealant is available from companies like Glasscages. The main drawback of restricting your options to silicone with the 'Aquarium Safe' label is that this type of silicone is not always easy to find locally, and it tends to costs more than the equivalent product, which can be found at your local hardware store.

What brand should I use?

There are many brand names under which silicone is sold in the USA. As long as you make sure that what you buy is pure silicone, and you let it cure completely before exposing your fish to it, they will be fine. Since GE Silicone is probably the brand aquarists will most often encounter when shopping in big chain stores such as Lowes, Home Depot or Walmart, below we will discuss different types of GE Silicone in some more detail.

Not for Aquarium Use - what does this label mean?

GE Silicone I for Door and Window is a type of GE silicone that has been used by hundreds if not thousands of aquarists with excellent success. Many years ago, this product even used to carry the label 'Aquarium Safe', but it was dropped, and in some cases replaced by a label 'Not for Aquarium Use'. This has caused significant confusion and concern among aquarists. Since the ingredients of GE Silicone I for Door and Window never changed, and many people continue using it to this day, there is no rational reason to believe this product has lost any of its suitability for aquarium purposes.

GE Silicone I vs. GE Silicone II - what's the difference?

Silicone I gives off acetic acid while curing, which causes a strong vinegar smell. People working with Silicone I many years ago in aquarium building companies tend to still have problems from the damage it did to their sinuses and lungs. If you only reseal one or two tanks per year you are probably fine, especially if you work in a well ventilated area, but the hazard involved with workers being exposed continuously to these vapors prompted the development of Silicone II. It gives off ammonia instead. It either is less damaging for people, or it smells so disgusting that folks are forced to wear breathing masks or take other precautions. Silicone II has about 20% less strength than Silicone I, but this is still plenty strong enough for building a tank. In many areas, Silicone II seems to be more readily available in colors like black and brown.

Once Silicone I is cured it no longer gives off acetic acid, and once Silicone II is cured it no longer gives off ammonia. In either case, it is very important to let silicone cure completely, but since ammonia is a much stronger fish toxin than acetic acid, this becomes even more of an issue with Silicone II. Once cured, neither Silicone I nor Silicone II pose a health risk to your fish. GE Silicone I for Door and Window has been used by so many aquarists with excellent success for so many years that there can be no reasonable doubt that it is aquarium safe.

Personally, I have installed rock backgrounds in a 29G tank and a 240G tank using GE Silicone II for Door and Window. These backgrounds consisted of real rocks that were attached to the back glass of the tank using silicone. Due to the irregular shape of the rocks, copious amounts of silicone were needed to complete the project. If memory serves, I used about 5 cartridges of silicone for the 29G, and more than a case of silicone for the 240G. Since the silicone is applied very thickly in places, many days should be allowed for complete curing, but in both cases no ill effects on fish, plants, or invertebrates were observed in many years of operating the tanks - at the time of writing this article (Jan 2013) almost eight years for the 240G.

Door and Window vs. Kitchen and Bath - potentially the big difference!

Silicone for Kitchen and Bath tends to contain biocides, which are toxins that leach into the water and prevent the growth of mold, mildew, and bacteria. Silicone for Kitchen and Bath is not pure silicone, but a silicone product with additives that act as biocides. In a 2011 press release, GE lists only its Kitchen and Bath Silicone as allergen fighting, biocide containing, with five to ten year mold-free protection. These additives have been blamed by many aquarists who have experienced fish loss after completing aquarium projects with silicone. Both GE Silicone I and Silicone II are available with biocides (kitchen and bath) and without biocides (door and window). Personally I have never used silicone with biocides for aquarium purposes, and I would not recommend it.

Until recently it was fairly easy for the aquarist to select an aquarium safe silicone by avoiding any product that was labeled as mold and and mildew inhibiting. Unfortunately these seem such irresistible properties for the homeowner that GE has begun to market all of its silicone products with some sort of anti mold message, not necessarily because they leach toxins, but because they form a smooth and durable surface that prevents the formation of mold. For example GE Silicone II for Door and Window now carries a 'mold-free product protection'. Although I have not tried the latest iteration of that product in a fish tank, I believe it is still aquarium safe.

GE has also caused some confusion amongst animal keepers with it's BioSeal label. The jury is still out on whether this refers to biocides or simply a superior surface.

100% Silicone - does this label mean I have pure silicone?

Silicone with biocides contains more than 99.99% silicone and only a tiny little amount of additives - enough to kill all of your fish, but not enough to stop the product from being marketed as 100% silicone!

The 100% silicone label is used only to distinguish silicone from siliconized-acrylic caulk. Silicone is an adhesive that can also be used as caulk. In fact, it has a number of properties which make it very desirable as a caulk - permanently waterproof, flexible, shrink-proof, and crack proof. Acrylic caulk and siliconized-acrylic caulk are far cheaper than silicone, and serve the homeowner for various sealing projects. Those caulks lack the adhesive strength of silicone to hold glass tanks together. They are generally not useful for aquarium purposes where we require an adhesive, not a caulk.
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