I came into a 10 gallon fish tank about a year ago. I came into it because it was cracked. No shortage of cracked fish tanks in the world. If you know how to repair them then you can build an elaborate system of scratch and dent tanks in just a few years1. I come across abandoned and broken-ish tanks at least once a year. And I always want to grab them. Until I moved to Baltimore I didn't have the room. But when I saw this one cracked tank in Grandma's barn I had dreams of a hydroponics system dancing in my head. The tank sat in my basement until about a few weeks ago when I decided to start fiddling with it. But I had only the vaguest idea how to repair it. And I'm betting you have the same issue if you're reading this2. And you're finding that when you start looking for "how to repair cracked 10 gallon glass fish tanks" (consider that a shout out to the Google webcrawler) you find a couple of guys filling their fish tanks after patching them and never telling you how they did it.
Before we start, I want you in the right mindset. If I have a crack in my wall I paste over it, smooth it, and paint it. It doesn't need to do more than look good. But here you need to have something that will hold back the pressure of 10 gallons of water or more. No problem? Go pick up 10 gallons of milk and tell me that's nothing. The patch I'm about to describe will go on the inside of the tank. If it were on the outside you'd have pressure from two pieces of glass pushing out at one point, right at the crack. The patch would either break or tear off. But if the patch is on the inside then the pressure is over the entire surface of the patch and all the surface from around the cracked glass is giving the patch extra strength instead of working against you. Or, how about this? If you were trapped in a big barrel with it's one hole closed up, would you rather have to reach through the hole and push off the patch or have to push the patch through the hole? Having the patch on the inside is better for keeping stuff in.
You're probably hoping that all you have to do is get some aquarium sealer and give the cracks a good once over. Alas, that stuff is flexible and will give the broken glass the wiggle it wants. You need reinforcement, first. Find yourself a piece of glass you can cut up. And you'll want to get a glass cutting tool. Available now at your local hardware store3. While you're there you need to get a tube of some aquarium friendly glue/sealer/caulk stuff. Silicone Aquarium Sealant most likely.
I had two cracks running from the bottom corner up the back of the tank reaching toward the middle. So I may have some bias towards my own personal fix. Deal with it.
You need to cut out a rectangle of glass. How big? Big enough to cover the crack. I don't mean cut out a piece of glass an inch wide and twelve inches long and place it over the crack at an angle. I mean measure how far it is horizontally from the far left point of the crack to the far right point. Add an inch beyond the crack. Add two if you like, but at least one. Do the same vertically from the top of the crack to the bottom. Since the crack probably runs clear from top to bottom that just means your piece will have to be the same height as the tank. If the crack hits or even gets close to the edge of the tank then you'll want your patch to go flush up against that side. Same with the bottom of the tank.
I couldn't mark my glass worth anything. Instead I measured and put a T-square on the glass. Then I pulled the glass cutter down the glass using the T-square as a guide. Ideally, you want to have some experience with glass cutters. I have and I still find them nerve racking. Not the sound they make as they score the glass (not fork scraping a plate irritating, but still enough to make your teeth itch). I mean actually breaking the glass. You keep applying more and more pressure and expect the glass to break at any second. If you're lucky you have a lovely piece of glass. If you didn't it could break anywhere and possibly throw sharp flakes around.
Oh! Are you wearing glasses? You should be wearing glasses.
If you did score the glass right you should be able to break the glass along the groove you cut. Please tell me you can see the cut. The whole thing? End to end? If not, do it again. Although, if you press too hard the glass will start to flake along the groove. Still, better it flakes than breaks in the wrong place. Dad has what are basically pliers with rubber grippers for doing his stained glass stuff. I had to put the glass on a table, put the score at the edge (facing up), and push down. And it worked! Alas, my second cut was less successful. I had much less glass hanging over the edge of the table and couldn't get the leverage. That's where I wanted Dad's pliers. Still, the break ran along the general area plus or minus a quarter inch. I just had to be sure not to touch the jagged part. Do I need to explain that jagged glass is bad? No? Fantastic. You're smarter than most of the neighbor kids.
Now, get a razor and cut away the aquarium sealer in the corners of the tank where you'll be putting your patch. In my case I stripped out the whole corner from top to bottom and about half way along the bottom. Dig for it if you have to. I want you to be able to stick the corner of your razor (or a toothpick or something pokey) into the corner and push it along without resistance. Remember hopping on your first skateboard and faceplanting over a grain of sand? You want to make sure that grain of sand isn't hiding in the corner of your fish tank. Your piece of glass should sit flush along the bottom or edge. Cut away everything that prevents that. You can wrap part of the razor in tape if it makes it easier to hold. Double, triple, and quadruple check for debris, sand, old gravel, whatever. I did, and I still found colorful bits of rock after I sealed everything up.
Now put the aquarium glue on one side of your piece of glass. Be generous. You're looking for something that will hold and will also look good. So place the beads of glue close enough together so that when you press this to the side of the tank there won't be air bubbles left between the pieces of glass. I mean, you can leave some. I left quite a bit, actually. But it'll look better if you get rid of the air bubbles. And if you can't get rid of them, you at least want there to be more glue than air. When putting pressure on it, don't just push from one side. Push from both. Or, set it on a table and put something under it for support. Your tank likely has a top and a base that prevent the glass from sitting flush on the table. You want something that keeps you from breaking the glass further by pushing on it.
Once you're happy with how your patch is sitting, you need to seal the edges. Take the Aquarium Sealant and put a bead along the edge of the glass all the way around. If you have rubber gloves you can use your finger to smooth the bead and make sure there's no holes for water to get through. Just make sure that there's no way for water to get between the panes of glass. If you broke one edge, make sure you're covering the sharp bits, too.
I put a bead down the outside of the crack, too. Partially for support, but mostly I wanted to cover the crack so no flakes would come off and I couldn't accidentally cut myself on it in a raging fit of incompetence later on. I used the razor as a putty knife to spread the bead, push it in, and smooth it out. It's obviously still there. It's just safer. I'm not making a pretty tank, here. I'm making a beginners level aquaponics system.
Let sit for 24 hours. Heck, let it sit for 48. No need to push our luck. You'll want some kind of moisture indicator under the tank. Newspaper or paper towels or tissue paper or a layer of flour should work. Even if you only lose a drop and it dries before you look again the water mark should remain.
What to do if you have a leak
If you have a leak you need to make it visible. To do this I put red food coloring in the tank and started filling it with water. I thought that I had to build up the pressure before it leaked, because it only leaked after around 2 gallons were added. No, dipshit, that means the leak is between 1/10 and 2/10 up the side of the tank. I could see red water between my patch and the original glass and I could see where it left the tank, but I couldn't see how it got behind the plate. So I dumped the water out, put the fish tank in the bathtub, and started filling up the bathtub. Once there was a couple of inches of water I saw the water rush in the bottom of the patch, fill it part way, and burble out in the corner about two inches up. I hadn't sealed properly. I made a bead, but hadn't used anything to press it down. It shouldn't have needed that, but now I know better. And now I know where the leak is. Once things dry out again, I can close that up.
Another way to narrow it down is to add one pitcher of water to the tank, and wait. Mark the water level with some masking tape or something and do it again. When it starts to leak you know the leak is between those two pieces of tape.
Here is what I'm going for in the long run.
See also https://www.kickstarter.com/projects/aquasprouts/aquasprouts-aquaponics-self-cleaning-aquarium-and
1 The fish store I've been visiting for supplies sells damaged and used tanks cheap.
2 Note to regular readers: I love ya' but my most popular pages are those where I'm telling people how to make, fix, or harvest something. This page is likely to be getting hits for years.
3 Did you read that last sentence in The Voice? No? Then read it again!