I cracked a plastic water tank for my humidifier, a Holmes HM-2060W. It's not a bad crack in that there is not a large gap, but it's 3-4 inches long and leaks water, which spills onto the floor, so it has to be fully sealed.

I dug up four different caulks/sealants I have lying around, and I think none of them are right? I'm concerned they'll leak when the tank is full.

  • Gasket & Seal: "adheres to plastic" but "Water Exposure: 3 hours"
  • Kwik Seal: "Not for continuous underwater use"
  • Tub & Tile: "Do not use below waterline"
  • Dynaflex 230: "100% waterproof" but "Do not use below waterline"

Can I use one of these? If not, what product should I get?

The humidifier:

Holmes humidifier

The sealants:


1 Answer 1


None of those.

100% silicone caulk. You potentially don't even have to worry about it being aquarium safe, but going there would not hurt. It literally holds all modern glass fishtanks together, so you know it can take water exposure, below waterline, just fine.

I repaired a crack on a plastic washing machine tub (which I'd otherwise consider to be an "un-glue-able" plastic) using silicone caulking and fiberglass mesh drywall tape (as a reinforcement, embedded in the caulking.) It's held for years. And I've repaired fishtanks with the proper aquarium-safe stuff (other kinds can kill fish.)

The other possibility, but it requires sorting out what type of plastic your tank is to find a suitable solvent, is to solvent-weld the crack.

  • I mostly agree, but there are products that bond better and stay more flexible than silicone. Dynaflex might be one of them. There's a reason silicone isn't the best choice for rain gutters, for example.
    – isherwood
    Feb 5, 2023 at 23:04
  • What are the things I listed? It's so hard to tell what they actually are. For example, Dynaflex 230 is called "elastomeric sealant", but the marketing language says '"Silicone Tough" latex sealant'. Is it not silicone?
    – dfrankow
    Feb 7, 2023 at 0:18

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