AQUARIUM REPAIR & SILICONE APPLICATIONS; What Silicone to use and what not to use, Aquarium Repair (& leak identification), Aquarium construction.
Updated 7/09/09

*Also how to use silicone to repair an aquarium (three different applications)
*DIY Aquarium Glass Thickness recommendations.
*YouTube Video demonstrating application of silicone for aquarium crack and hinge repair


SILCONE BASICS:

Silicone rubber is a unique synthetic elastomer made from a cross-linked polymer which is reinforced with silica. Its characteristics are such that it provides a balance of mechanical and chemical properties useful for bonding glass aquariums (not acrylic).
The basic formula is: Me3SiO(SiMe2O)nSiMe3

RTV Silicones are the silicones commonly used for glass construction and is often sold in Home Improvement and Pet Stores (or online)
RTV Silicones consist of polydialkylsiloxane with terminal hydroxyl groups (groups made up of an oxygen atom and a hydrogen atom connected by a covalent bond), which are reacted with organosilicon (chemical compounds containing carbon silicon bonds) cross-linking agents. This operation is carried out in a moisture-free environment and results in the formation of a tetrafunctional structure (A chemical structure that possesses four highly reactive sites). Curing takes place when materials are exposed to moisture. Atmospheric moisture is sufficient to trigger the reaction, and thickness should be limited if only one side is exposed to the moisture source. Curing is also relatively slow, reliant on moisture ingress into the polymer which produces Acetic Acid vapors.

Different formulas and (combined with temperature and outside humidity) will affect cure time. Dow Corning and GE have proprietary formulas (although both are packaged under other brands such as DAP & All Glass Aquarium) that cure at an even slow rate as compared to some cheaper formulas, especially used in construction where this is not an important factor. This however is important for aquariums to have an even and slow cure time as I have used 100% silicones such as CRL that would cure too quickly resulting in a less than desirable bond.
Also some formulas such as GE Silicon II have a mildew inhibitor (which would be toxic to the fish).

100% RTV Silicone Rubber Sealant produces a flexible, durable weather resistant seal between similar and dissimilar materials; RTV Silicone provides excellent adhesion to tile, glass, porcelain, ceramic, fiberglass, non-oily woods, plaster, painted surfaces, metal, many plastics, and rubber. It will withstand temperature extremes from -60°F. to +450°F. and is waterproof for freshwater or saltwater use.

It is noteworthy that plastic (which is made from oil) is not mentioned. RTV silicone is not useful for bonding acrylic aquariums and the oils within acrylic will eventually cause peeling away (lack of adhesion).
The bottom line is do NOT use silicone for repair of Acrylic/Plexiglass aquariums (or other acrylic applications)!

For Plexiglass/Acrylic aquariums you would need to use Plexiglass or PVC Cement (Glue) which works by “melting” the Plexiglass panels together. The is best applied with a needle applicator between the Plexiglass panels along with applying pressure. Silicone is often used after initial Plexiglass construction to provide more water tight seal along the seams of a Plexiglass/Acrylic aquarium, but this will peal after time (it can be re-applied) and provides NO strength for the actual plexiglass panels.
Silicone can also be used to form a seal where water pressure will push the glass panel against a surface that it would otherwise would not adhere well to (although once pressure is removed after initial filling, the silicone will likely not “seat” again in such an application where silicone does not properly bond).


AQUARIUM APPLICATIONS:

RTV Silicones are useful in the construction of glass aquariums and sealing the joints in DIY wooden aquariums. Silicone is NOT useful for acrylic aquariums as it will peel away over time due to the inherent oils in plastics.
Not all silicone formulas are the same, as stated earlier, Dow Corning and GE produce a RTV silicone that cures at an even and slow rate necessary for aquariums. Why is this important? Too fast a cure will not allow proper surface adhesion especially in important seams. It is also hard to work with in building aquariums and the person needs to work very quickly (usually too quickly) for proper aquarium construction.

Also you want to purchase pure 100% RTV Silicone as many (including ones produced by Dow Corning) have other ingredients for different applications such as fiberglass.

A good example of a silicone you do NOT want to purchase is Dow Corning Marine Sealant which I have actually seen recommended for aquariums! This product is great for fiberglass hulls of boats however it lacks the proper strength for aquariums due to added ingredients that allow for adhesion to plastic.






AQUARIUM REPAIR;

With a good silicone, you can perform a few repairs yourself.

The picture/diagram to the left shows what to basically to look for as to silicone in the important joint where two glass panels meet. This is where most of the strength is obtained from the silicone. If the silicone is missing or pulled away (as can be easily observed by looking through the glass at each joint to see if air or water is present in place of silicone), you may need to repair this. The silicone in the inside corner gives added strength and small leak protection, but is not as important for aquarium strength as the silicone in the joint. This is also important that braces (if your aquarium has a brace) have this silicone as well.
(please click the picture for a better view

Small Leak Identification:

Often small leaks are the most difficult to find (identify the source of the leak so as to repair). I have been called out by a client who thought that their tank had a leak on the bottom (which would require a strip down of the tank and stripping of silicone and reapplication), only to find that the aquarium actually was only seeping water from the top (often under the trim) and this small bead of water would run down the side seem in a manner that went unseen. This type of leak can be found more easily by gently running a paper towel along the sides to look for trace amounts of water which the paper towel would quickly find.

If this is your leak, this could be from the trim (which is often a plastic material and does not adhere to the silicone as well and can often be repaired by simply lowering your water a few inches and stripping the silicone around the trim and then replacing this silicone. Sometimes simply adding silicone in gaps in the trim or even lowering water level or reducing condensation of water on the glass top which may find its way around top gaps in the aquarium and then to the bottom of the aquarium (often resulting in a wet stand or similar condition)

For an older tank that is starting to leak (no broken glass), this can be repaired following these basic steps:
[1] Empty aquarium
[2] Remove ALL old silicone with a razor blade (I prefer a single edge razor blade, see picture to the right)
[3] Clean and prepare surface with fine sandpaper.
[4] Make sure all surfaces are dry.
[5] Apply silicone (a caulking gun works best), making .5 (or smaller) cm. bead in the inside edges.
[6] Tighten all outside corners with strong reinforced dust tape. For tanks over 75 gallons I recommend wooden clamps.
[7] Immediately uses your index finger to smooth ALL inside edge beads into a smooth surface, quickly wiping away excess silicone.
[8] Your repair is complete in 48 hours.

For non structural corner chips:
[1] Drain aquarium down to the area of the chip.
[2] Inspect/ clean/ repair the inside area in needed.
[3] Clean out broken glass on the outside chip area
[4] Carefully clean out broken glass (gloves are recommended), a razor blade followed by fine sand paper works best.
[5] Inject new silicone followed immediately by a piece of glass that fits reasonably well into this chip.
[6] Add more silicone if necessary.
[7] Smooth repair and add a piece of wax paper (or similar waxy or mildly oily band aid), followed immediately by strong reinforced duct tape to hold this repair in place.
[8] This “band aid” can be removed in 24 to 48 hours and the aquarium re filled.

For broken plastic hinges:
[1] Remove old hinge
[2] Place both pieces of glass on a firm level surface
[3] Run a .5 cm bead of silicone carefully down the area where both pieces of glass come together.
[4] Immediately, using your index and middle finger, run these fingers over the top of this silicone bead forming sharp peak of silicone between both pieces of glass. Remove excess. Bare hands (fingers) work best as the oils present repel the silicone from adhering too much to your fingers. The silicone should look sort of like this after finished: /\
[5] After 24 to 48 hours your new and more durable hinge is ready.

For MINOR cracks
[1] Clean the area on BOTH side of the crack thoroughly with a razor blade (so solvents)
[2] Apply bead of silicone as described above and in the video below forming a ridge with your fingers evenly along the crack (adding a plate of glass on the inside side of the crack will greatly increase strength). Flip the aquarium and repeat on the other side.
[3] Allow at least 24 hours to dry, allow 48 hours is better.
[4] This repair is ONLY recommended for aquariums under 10 gallons with single cracks, generally on the bottom.

For MAJOR cracks (only single cracks, not “spider” cracks)
For larger aquariums with single cracks I recommend adding a plate of glass to the inside of the crack repair in step #2. This plate need only cover the crack plus an inch or two, not the full bottom or side (although that would certainly be better yet).
I generally do not recommend this repair for tanks over 100 gallons, although I have cut glass panels to fit over the entire bottom without replacing the old cracked panel for larger aquariums or multi cracked smaller aquariums.
Otherwise I STRONGLY recommend replacing the glass panel.

Here is a YouTube Video we made for aquarium crack and hinge repair (it also shows how to correctly apply silicone and cut glass)


FOR DIY AQUARIUMS
(or simply what to look for in a new glass tank):

Recommended Glass thicknesses:
(Length x Height x Width)

2 x 1 x 1 = 5mm glass
2 x 1 x 1.5 = 6 mm glass
2 x 1.5 x 1.5 = 8 mm glass
2.5 x 2 x 2 = 10 mm glass
3 x1 x 1 = 6 mm glass
3 x 1.5 x 1.5 = 8mm glass
3 x 2 x 2 = 12 mm glass
4 x 1.5 x 1.5 = 10mm glass
4 x 2 x 2 = 12 mm glass
6 x any size x 2 feet max. = 12 mm glass

Please note that the formula for gallon calculations is L x H x W divided by 231 = Gallons. Multiply this by 3.785 for liters.
I do NOT recommend a glass aquarium over 250 -300 gallons!

Trim
• Please keep in mind that when either building or purchasing a glass aquarium, the thickness of the glass (which should be float glass), along with the type of silicone used, and finally construction methods are what will basically determine the quality of the finished aquarium.
The trim (which is generally plastic) offers LITTLE support other than providing a “cushion” between the bottom glass and stand and as well a “convenient” way to place the lid, lights, etc., on top. Of coarse the trim does certainly add to the attractiveness, but as someone that has worked with high end aquarium manufacturers and built/repaired many myself, I can tell the reader here from an experience that the trim offers little in support and if it is necessary for support, you have a dangerously poorly built aquarium!
As well, I have removed trim from many tanks (including large ones) and filled them with water with no difference other than the unattractive aspect.



SUMMARY:



Can you easily purchase the proper silicone at a Home Improvement Store? The answer is yes, just do not purchase anything other than 100% Pure Silicone (color does not matter) and I strongly recommend brand loyalty as there are good reasons behind GE and Dow Corning patents. To be safe, if you are unsure of the silicone you are buying, do not try and save a few bucks at the local hardware store when your local Pet Store or Aquarium Store have the correct product for maybe a few dollars more (unless an exploding aquarium in the middle of the night is your idea of excitement).
If you can not find quality Aquarium Sealant (either Dow Corning or GE, or one of their repackaged brands such as DAP or All Glass Aquarium; which hold patents for a good reason), please click on the picture to the left for an online source.