A lot of people will just throw away an aquarium when it begins to leak or possible gets damaged. Now Iím sure your local pet shop would love to sell you a new one to replace the one that got cracked or just started leaking but with a little time and a few dollars you can repair the aquarium that has served you quite well for all of those years.
All modern glass aquariums are constructed out of glass panes, plastic frames on the top and bottom, and silicone sealant which holds all this together. When purchasing the silicone sealant make sure it is 100% silicone sealant and that it has no additives. Do NOT buy silicone that is labeled for the bathroom, this has additives in it which keep it mildew resistant. These mildew resistant additives are quite deadly to your fish. Most pet shops carry silicone that is intended for aquarium use. You can buy either a small tube (small repairs) which holds a few ounces or a more economical way is to get the silicone in a large tube (large repairs) that fits into any standard caulking gun.
The best way to remove the old silicone from the corners is to use razor blades used in the common utility knife found at hardware and home improvement stores. There is many ways to use the utility style razor blades. The safest way is to grip the top side of the blade, not the sharp edge, with special pliers called "vise grips". Vice grips are adjustable so they will keep whatever it is holding on its own. Regular pliers will do just fine but your own muscles will be exerting the constant pressure to hold the blade firmly, this can become tiring on larger repairs.
If your tank just has a leak this repair is the simplest of all to do. If you are able to locate the particular seam that sprung the leak just remove the silicone from that seam only. To remove the silicone with the blade mounted in pliers position the blade flush with the glass, this acts as a guide, and cut into the old silicone until it bottoms out into the adjoining side of glass. Cut along the length of the glass from one end to the other. Now do the same to the opposing side. After cutting in the appropriate corners the old silicone should come out in a strip. Any residual silicone in the area of repairs can just be scraped out using the same technique keeping the blade flush to the glass surface.
Silicone when first applied gives off somewhat of a pungent odor while itís curing for the next 24 hours. The odor smell like sour vinegar. Unless this scenario doesnít bother you it is advisable to apply the silicone in a well ventilated area (i.e. garage, outdoors or basement).
Now you can apply the new silicone. When using a small or large tube the technique is the same. Keep the end of the tube touching both sides at the same time, apply enough pressure to the tube and lay a bead of silicone down the length of the area being repaired. Some finesse is needed here so that the bead laid is relatively smooth and not to "bumpy". Now wet your finger in water, this will somewhat keep the silicone from sticking you, and place it in the corner so youíre touching both sides and smooth out the silicone from one end to the other. The more you use silicone, like everything else, youíll get better at applying it.
Let the silicone cure for one day. Fill your tank and check for leaks. No leaks, youíre done. Leaks? Back to square one.
When replacing a piece of glass that is actually cracked or broken you will have to disassemble your tank. If the tank has a lot of age to it sometimes the plastic top or bottom can simply be pulled off with a little effort. If the tank is newer and the frame has a solid grip you will need to use your razor blade and work it underneath the edge between the glass and frame. Several passes at this is required to reach the end of the glass where it meets the frame. This should be done inside and out if working on the top frame and from both edges if working on the bottom frame. Once this is done cut out the old silicone from the appropriate corners on the inside of the tank. There is an ever so thin layer of silicone between the joining edges of the glass pieces, this is for assembling and sealing purposes. If the glass panel is broken in many pieces usually just pivoting the broken pieces will break this bond. If the glass is just cracked you will have to slide your razor blade into the joining pieces and use a little pulling pressure while making a few passes at it. Starting at a corner works the best. Make sure all residual silicone is removed. Any small pieces of glass still attached to the frames can easily be removed using a flat sided screwdriver. Once all the old is removed do some measuring, donít forget the thickness, and go to glass shop and get the new piece that is need. Shop around prices do vary quite a bit sometimes.
When installing your new piece of glass apply a very small amount of silicone to all the edges and put it into place. Reinstall the frames using a liberal amount of silicone, any excess should be removed after curing with a blade. If you try to remove the excess that may squeeze out around the frame or glass before curing it will smear. Next apply silicone to the interior corners as described above. Let this cure for a day and then leak check your work.
If you arenít worried about aesthetics and may have just a cracked tank that you are using for what ever purpose then keep in mind that a bead applied to the interior side of the crack and slightly leveled with a finger leaving a little thickness (1/16-1/8 ") usually will fix the leak. I personally have a 25 gallon tall that is cracked from bottom to top on a side and has been repaired in this manner and it doesnít leak.
Another method of repairing a cracked or busted tank is to buy a piece of glass that will fit against the old on the inside and seal the corners accordingly. If itís the bottom gravel will hide it. If itís on the back side of a tank try using a piece of tinted glass to hide the broken piece. Now best of all if itís on the side you get the "mirrored effect" when looking through the front and wonít be able to see the damaged piece.
You may wonder if itís worth it or not but to give you an example I found a 46 gallon Euro stlye tank ( $120 retail ) in somebodyís refuse container with a broken out back. This tank was brand new with a sticker still attached. Being brand new the top frame I agree was timely to remove but with a total of five hours of my time and $15 for a piece of glass I have a beautiful "new" tank. Hey itís new to me!
I can not stress this enough so PLEASE PAY ATTENTION while working with broken glass, I know itís common sense but Iím saying it anyhowÖÖPlease take your time and be careful glass cuts painfully fast and deep so wear some thick gloves while removing the broken pieces. I want all who read this to tell me what a fantastic tank that they fixed for next to nothing. I donít want to hear my friend Bubba say: "Hey Jim look at my tank I repaired it only cost me 20 stiches!"