Cracked bottom of tank, would like advice on repairing it..

I have a 29 gallon tank that I have owned for about 4 years now. I had goldfish in it about 4 years ago and kept them for 2 years and everything was great until they all got sick and passed away.

I put the tank in my basement and forgot about it. My friend wants to give me her fish that were left behind from a renter before she flushes them.

I then bring the tank upstairs and set it up on the tank stand only to discover that it has a single crack on the bottom.

It is a typical 29 gallon tank. I went looking for a replacement but they are all around 70+ dollars for an equal replacement tank, a bit more than I can currently afford. So I have been researching repairing the tank. The crack is a clean crack, no missing pieces.

I figure I can repair it by having a piece of 1/4" glass cut to the dimensions of the inside of the tank. I will then lay the glass inside the tank and seal it up. I am a strong do-it-yourselfer so I am not worried about doing the work, I just need advice on the proper repair and preparation.

Should I remove all of the existing silicone around the bottom edges of the tank so the new glass can lay flat on top of the existing cracked glass with no gap and after the new glass pane is in place, then lay down new silicone? Or should I run new beads of silicone on the bottom edges and then lay the new glass down on top of it and have a tiny space in between the original cracked bottom and the new replacement bottom?

What is the best way of repairing this tank? I can get a new piece of glass cut for about 20 dollars and new sealant for around 5 dollars so this is less than half the cost of replacing the tank.

I have not seen any cheap 29 gallon sized tanks on CL. CL has either tiny tanks or huge tanks.

 

Hello; I have done similar repairs with success in the past. It is difficult to know exactly the best approach without seeing the crack. My guess is that the crack runs to the edge of the glass at two places.
There are some potential issues with either way of adding a patch.
If you leave the old silicone in place the new silicone may not adhear to the old well. I have had some luck but also some failures with this. A key may be to clean and scrub the old silicone very well.

Another possible issue with leaving the old in place is that the new glass may not lie flat against the bottom but may rest on the old silicone and leave a gap. This could lead to additional leaks and if the new glass is thinner than the tank glass it could crack.

Cutting out enough of the old silicone to allow the new glass to lay flat on the botton would be my choice. The issue here is that there is some potential for making the existing crack worse with the pressure you may apply when cutting out the old. I would use a utility knife with very sharp baldes and go slow near the crack, perhaps supporting the bottom glass if possible?

Making the surfaces clean and dry should help the new silicone to adhear. It will likely be a messy proceedure and may not look so nice. I have a 20 long repaired this way that has been in and out of use for many years.

I will repost and old set of instructions I wrote about sealing a tank. And also there will be some added risk of cutting your self, so take care and wear eye protection in case the glass chips fly.

 

Hello; Here is a revised old post about sealing a tank with silicone. Resealing a tank can be a chore and may not work the first time, but if you get a good seal it should last for years.
I have found that trying to run a line of silicone only on a portion of a tank has often not worked out very well. If a tank leaks, I find it best to reseal all the corners and sides at the same time.
-(In my experience the silicone needs to be applied in all corners and sides while it is still tacky so that it will bond to itself at the apex of each side.)
-(New silicone does not seem to bond to old silicone very well)(Some have posted that it will stick better to the old if the old is well cleaned, I have yet to test this myself.)
I also cut out the existing silicone with a utility knife from all corners before trying to seal with new stuff. I find a shop vacuum a handy way to remove the bits of silicone. If the first application of new silicone does not seal the leak, I cut out all the silicone that I have just applied and seal the whole thing again. I also find that the silicone tubes that fit in a caulking gun to be the handy way to apply silicone. The tubes of silicone that you squeeze with your fingers are just too hard for me to control and I have not tried to use them in years. You can plug the end of a big tube of silicone with a nail or something round to keep it from curing too quickly, as you may need it again.
-(Note – When you cut the end of a silicone tube, make the first cut near the end so as to have a small opening. You can always cutoff a little bit more if the bead of silicone is too small.)
Some additional pointers. I usually give a tank a good cleaning before applying the new silicone. I use a paste made from ordinary table salt (Without iodine) and a soft sponge.
-(I do not use a sponge or cloth that has been used with soap or chemicals.)
-(I have buckets and sponges that are used only for my tanks.)
The salt paste does not scratch glass and does a fair job at removing the deposits from the surface of the glass (be sure the sponge or cloth used to apply the paste with is not coarse enough to scratch the glass on its own and watch for bits of gravel that seem to jump into the salt paste.) (Letting the salt paste dry to a haze should desiccate most critters if you want to disinfect a diseased tank.)(Note- wetting the glass will soften up the deposits somewhat.) (Also for stubborn mineral deposits, a razor blade in a holder that gives a firm grip is useful. Use a shallow the angle on the blade and keep an eye on the corners of the blade. The corners sometimes get bent and will scratch a thin line. Rinse the salt paste out and let the tank dry well.
Once the tank is clean, I think it needs to be very dry before applying a bead of new silicone. Dampness tends to hide in the very small seams and in the corners of a tank. I suspect that dampness prevents the silicone from bonding properly to a surface and may result in a leak in a good looking seam. It can take a while for the moisture to dry out of the small seams.
(note- this is not real critical, but try to have the tank in a place where it can sit for a day once the silicone has been applied, so you do not have to move it until the stuff can cure some. Silicone labels often say that it will cure enough in two days to add water. (While I have gotten away with around a two day cure with thin beads in small tanks it has not always worked. It seems prudent to wait a few more days to be sure. Some experienced people have suggested seven days for a full cure.) Thicker beads of silicone in large tanks will likely take the additional time to cure. Silicone will skin over and harden from the outside in. Push on a thick part of a bead with your finger to see if it feels firm and solid.
If you manage to lay down a decent bead of silicone try to resist poking at it in an attempt to smooth out a bump or irregular spot. I usually only try to smooth out the glob where the corners meet at an apex. You may get away with wetting your finger tip lightly with water and gently touch it. (Water on the outside of a joint should not cause a problem if you use just enough to dampen the tip of the finger and not enough to drip off.)(Does it sound like fun yet?). You will want several paper towels or cloths handy to keep the silicone wiped from your fingers. It will get on everything if you don’t and it is a real pain to clean off. If you use a cloth to wipe silicone the stuff does not come out, so do not use a favorite towel
If you happen to mess up and need to clean up a smear,( and find that the more you wipe it, the more it smears.) it may be easier to let the silicone set up for a day or two so that it is no longer gooey. It can then be rubbed off with a cloth or your fingers.
I try to test the tank in a place where a leak or having to empty the tank will not be a problem. I also allow a resealed tank to sit full of water for a few days to be sure the tank does not leak before moving it into the house. This may not be wise or practical with a large tank as moving may open up a seam. Good luck with whatever procedure you decide to use.

 

Thanks for the pointers. I would really appreciate those instructions.

I do have another question, should I have the glass cut to fit perfectly inside the tank or should I give about 1/8 of an inch leeway so there is about 1/16 of an inch gap around all 4 sides?

 

Thanks for the pointers. I would really appreciate those instructions.

I do have another question, should I have the glass cut to fit perfectly inside the tank or should I give about 1/8 of an inch leeway so there is about 1/16 of an inch gap around all 4 sides?

Hello; A 1/16 inch gap all around could be filled in by the silicone and should help in fitting the glass. This will likely be a good way to go. These types of projects often present unexpected problems, keep an eye out for an unexpected issue.

Two additional thoughts. 1) I would (and have done so) smear a gob of silicone directly over the crack itself. 2) for the test fitting of the new glass some sort of handle may come in handy; a suction cup or perhaps a wooden knob siliconed to the glass?

Try to think everything out before adding the silicone as these things are sometimes a one shot deal. While you may be able to pull the glass, something may well break and the clean up of smeared silicone is a chore. Good luck.

 

HI everyone. Well I have an issue. I found a place that went and cut me the glass as needed. I called my son at home while I was at the glass place and asked him to measure exactly the dimensions. It turns out that he measured the width of the tank incorrectly. The actual width of the tank was 12 1/4 inches wide and he told me 12 inches wide. So I had the glass cutting place take off 1/8 inch so I would end up with a gap of 1/16th on both sides. well now I have a gap of 3/16ths on the width. The length of 30 inches was correct so I still have a 1/8th of a gap on the length.

He also under measured the thickness of the glass and told me it was quarter inch but it actually was 3/8ths of an inch so I figure in the end, I now have a 1/4 inch gap on the glass on both sides of the width.

My question is this, if I lay enough of a bead of silicone in that 1/4 inch gap and fill it completely, should it not leak? I'm just wondering if 1/4 inch gap is just too much for silicone to fill. Remember, the patch of glass will be sitting on the bottom of the tank on top of the existing tank.

I am going to go get another pleice of glass cut if needed but I really don't want to if I can help it.

 

I would either replace the entire bottom or scrap the tank. Cracks on the bottom cannot be trusted to a simple silicone seal. Check out any local flea markets, craigslist and such for a basic tank. What kind of fish are being rehomed anyways? You may not need a 29gal but you may also need a bigger tank

 

I am not sure if you read my post but I am adding an entirely new bottom to the tank. I'm not removing the old bottom but instead I am laying down a new sheet of 1/4 thick inch glass on top of the existing bottom. The problem that I am concerned about now is will a gap of 1/4 inch on the long sides be too much to be filled by silicone. I originally wanted a gap of only 1/16th of an inch but my son screwed up on measurements so the gap is quite a bit more. The strength will still be the existing bottom of the tank which only has a single crack, it isn't broken out.

But I am definitely not doing a simple silicone seal fix.

 

I am not sure if you read my post but I am adding an entirely new bottom to the tank. I'm not removing the old bottom but instead I am laying down a new sheet of 1/4 thick inch glass on top of the existing bottom. The problem that I am concerned about now is will a gap of 1/4 inch on the long sides be too much to be filled by silicone. I originally wanted a gap of only 1/16th of an inch but my son screwed up on measurements so the gap is quite a bit more. The strength will still be the existing bottom of the tank which only has a single crack, it isn't broken out.


But I am definitely not doing a simple silicone seal fix.

Hello; I think that I understand the question. Unfortunately trying the glass you have is the only way to get the answer. My guess is that it should work based on doing similar repairs, but still a guess.

I would test fit the new glass without any silicone to see if it indeed lays flat on top of the cracked bottom. If it lays flat then being thinner will most likely not be an issue. (another educated guess) I have made patch panels of thinner glass than the tank glass. The tank glass will carry the stress and the patch has only to make a water tight seal. If you had multiple cracks the story would be different.

I would also see if the new glass can be slid over to cover the places where the crack meets the sides of the tank. This is where the patch will count. You may be lucky enough to avoid the larger gap near the crack. I have patched cracks with a replacement glass that only went a few inches beyond the crack itself. The edges of the patch glass away from the crack only have to be water tight.

This sort of patch has worked for me most times, but not always. I have a tank with multiple cracks patched in such a way. I eventually had to patch it on the outside as well as the inside. It is ugly but held water the last time I put it into use.

Perhaps the thing to stress most is this. Have everything ready to proceed quickly. Have the glass clean and ready to go on the tank and the patch. Have some cloth or paper towels handy to clean your fingers to avoid getting silicone all over the place. Use a fresh silicone tube not one you have plugged and allowed to sit around as it will likely have a hardened plug of cured silicone that will pop out when you are in the middle of the job. (This has happened to me a few times.) Once you start to lay down the lines of silicone keep on with it and get it down. It will begin to skin over fairly quick. Don't answer the phone or take a break. Lay the new glass onto the fresh silicone and push it into place. Once satisfied it is in place where you want it, run silicone around the edges on top of the new patch to give an additional leak barrier.
At this point it will be very difficult to remove the glass patch even from uncured silicone so have it planned out ahead of time. Once cured I doubt that the patch can be remover without breaking something. (I have some 1/4 inch thick pieces of glass about 3 inches by 10 that I siliconed together on the flat surfaces with about a four inch overlap. I siliconed them together back in the 1970's and they will not come apart. I tried prying two others apart a while back and broke them.)
Let it cure for several days (A week won’t hurt). Fill and test it somewhere safe. If it does leak when tested all is not lost. It can be sealed with more silicone but the job gets very ugly and messy looking. At least the crack is on the bottom.

Good luck.

 

Thanks for the advice. Well the new repair glass is actually 1/4 inch thick whereas the original glass is 3/16 inch thick so the repair piece is thicker. I'm pretty sure that a thick bead of silicone will fill in that 1/4 inch gap, I just wanted some reassurance. LOL