There are few things in life that are more frustrating for an aquarist than a leaking aquarium. It usually starts out with you noticing a drop or two of water hanging off the bottom edge of the tank, which soon develops into a drip...drip...drip, at which point you grab a handful of paper towels so you can start sopping up the water while you try to figure out where the leak is coming from.
Understanding how glass aquariums are constructed will help you find the place where the leak is originating from. When a glass aquarium is constructed, the structural integrity (what keeps it from falling apart) and water tight integrity are established where the glass meets the glass. If you look at where the glass panels meet, that bare fraction of an inch between the glass panels where the silicone is is where the rubber meets the road, so to speak. Water tightness and strength are minimally increased with the silicone that is spread out inside the tank where the glass panels meet. If the tank has been constructed properly and the silicone between the glass panel edges is flawless with no bubbles or gaps in the silicone, the tank will hold water without leaking.
Finding where the leak originated can be difficult. It seems like, nine times out of ten, it isn't where you are seeing the water on the outside of the tank. If the tank is fairly old, wasn't constructed properly, or hasn't been cleaned carefully over the years, you may notice the silicone sealant separating or curling up from the glass inside the tank. If there is some obvious damage to the silicone sealant in the tank, that would be a good place to start looking for the origination of the leak.
Sometimes the leak appears as a tiny stream of water shooting out of a silicone seam in the glass. If there is not obvious damage to the silicone behind the water stream, there is a simple way to find the origin of the leak. Fill a syringe with colored water (food coloring or ink works well). Insert the syringe needle into the silicone where the water is coming out and slowly depress the syringe plunger. You will be able to see the colored water enter the silicone and follow the trail of the "tunnel" that the tank water followed back to its origin.
Keep in mind that every aquarium leak starts from the inside of the tank. While you may be able to smear some silicone on the outside of the tank where the water is showing up, that won't stop the leak where it begins. Also, keep in mind that water can run a long way between the silicone and the glass in the tank seams before it finally appears outside the tank.
No matter where the leak starts from, in order to repair it you are going to have to drain the water down to below where the leak starts unless you decide to just smear some silicone on the outside of the tank where the leaking water is showing. You can also try to use a strip of duct tape to stop the leak. From our experience, neither of these fixes work for long, if at all. The main reason that resealing the tank from the inside, rather than the outside is that when the patch is on the inside, the water pressure presses the patch into the glass, whereas with the outside patch, the water pressure is pressing the patch away from the glass.
Get a siphon hose and start draining the tank an inch or so at a time. Keep an eye on where the water shows up on the outside of the tank and when it stops leaking, stop draining the tank. Follow the water level around the tank, looking for damage to the silicone in the tank, looking for silicone that is loose or other silicone damage. On the outside of the tank, mark the damage location with the felt tip marker. If you have determined that the leak originates at the bottom of the tank, drain the tank down entirely and remove the substrate. It may seem like a lot of trouble to go to, but in the end, you will find that it is easier to do that fighting the water and substrate contaminating the repair site.
At this point, you will probably want to move the fish and other critters in the tank to a temporary tank.
What You Need
- 100% Silicone Sealant without any additives such as mildew control.
- Acetone for cleaning the glass surface areas and your hands, later.
- Single Edged Razor Blades preferable in a holding tool.
- Paper Towels for cleanup and soaking up water.
- Felt Tip Marker for marking where the leak is appearing.
- Replacement Water to refill the tank when it is patched.
To start the repair, with the single edge razor blade, cleanly cut the silicone 1" around where the leak has originated from. Scrape all of the silicone off the glass without letting the blade go between the panes of glass. Clean the scraped area with a paper towel soaked with acetone and let dry.
When the repair site is dry, again, open the silicone tube and apply the silicone to the site, coming up over the old silicone on either side of the leak site. Smooth the silicone down and out with a piece of paper (a business card works great) or flexible piece of plastic. Don't just gob the silicone on the site thinking that a lot of material will do the job. Not only does it look ugly, it usually doesn't make a good seal.
Once you have finished applying and smoothing the silicone into place, allow the silicone to dry for 24 hours and then refill the tank with freshwater (no sense wasting sea salts if the patch doesn't work) and check for leaks. Let the tank stand for 24 hours to make sure that it isn't leaking again, then drain it and set it up again, just like starting over with a new tank.
If your repair didn't work you may have to strip the tank and Major Aquarium Repair.