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If my aquarium begins to leak, I know I could repair it by removing everything, applying silicone (or another sealant), waiting for it to cure, and then starting over.

If the leakage is at the top of the aquarium, can I drain just enough water to repair it, then re-fill after the sealant is cured?

Should I use regular silicone for this "hot" repair, or might it harm the fish?

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Silicone itself is harmless, as it is nontoxic, non-organic and pretty much unreactive.

During the hardening process silicone sealant will produce acetic acid, this causes the stench that smells like vinegar. Depending on the amount of silicone, you should not need to worry about some of that acid contacting your water (if you spill something), the amount should be too minimal to have an influence on your aquariums pH.

Some silicone sealants intended for use in bathrooms (or similar) might contain fungicides which may negatively affect your aquariums life.

Disclaimer: I have not tried to do this before, but everything I knew about silicone sealant told me that this would be safe and a quick search on Wikipedia confirmed my thoughts.

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    It doesn't work. The new silicon won't stick well to wet surfaces and will not stick to dried silicon in general. Best bet would be to use some superglue or aquarium safe epoxy, but both are just temporary fixes until the tank can be replaced or completely drained and repaired. – Jestep Jan 18 '18 at 21:20
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One problem you might run into is that new silicone will not stick to cured silicone, so you won't get a perfect seal just by caulking over the old stuff. I would also be concerned about the strength of the rest of the seam that's not leaking yet.

As for silicone, like Baarn says be careful about any with fungicides or other additives. Dow Corning 999A (clear) and 795 (black, I think) are both fish-safe, I believe. Another thing to look out for is the expiration date, since some types (particularly 795) won't cure if they're too old.

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Too many uncertainties here. Any structural joint needs careful preparation, correct materials & correct procedures. If you mess up you may lose your fish, a risk not worth taking.

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For a temporary solution: I had a small crack in my tank, and stuck some chewing gum on it until I was able to get a new tank a few weeks later.

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Yes you can do an external repair. Read below and take a look at the time lapse. If you cannot drain your aquarium to repair a leak from the inside, and you need to work from the outside, you have to beat hydraulic pressure. The way you can do it, is by patching from bottom (where your leak is) to top.

  1. Before you begin, drain as much as you can.
  2. Use a 2" / 5 cm tape (scotch / masking / duct / whatever).
  3. Patch this tape with a coat or 2 of a plastic bag.
  4. Put a stripe of silicone (any kind - since it's external). Now your "band-aid" is ready.
  5. Put this patch on your leakage in a way that the dripping is chased up. You cannot completely seal since hydraulic pressure is stronger than silicone. But the silicone is strong enough to push the water flow 1"- 2" (5-10 cm) up. Just make sure you chase the leak upwards - not downwards, and not sideways.
  6. Every 20 minutes or so, you can take of your patch (the plastic bag helps here), and put a new patch, 2" higher up (repeat steps 2-5) with a small overlap between the previous and the new silicone, so that you continue chasing upwards. If you miss (leak goes in another direction), repeat on this section, till you get it up.
  7. As you go upwards, at some point the hydraulic pressure will be low enough that your leakage stops completely. Even if you are not lucky, once you reach the top water level, the water pressure is 0, and your leakage will be over.
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If looking for a strong sealant, try the silicon sealant at West Marine. I have used the general type with the West Marine labeling (red and white tube) which is strong and has no toxics. I am not sure about the other types they sell which have varying hardening times and degree of hardness. You can also find the standard GE silicon seal (without fungicides) at Walmart for about four dollars per tube, which has been used by many who repair aquariums.

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