Why does my standing well water leave a film or gunk?
Minerals like iron, calcium, or manganese in well water oxidize and precipitate out when exposed to air, forming a powdery film or sediment overnight. This common reaction makes the water look murky and creates buildup on basins, but it is typically a harmless aesthetic issue rather than a sign of contamination.
What the numbers show
Counted from the threads quoted below. How we count
- The most recent one is from September 2026.
- This comes up 15.0× more often in r/HomeMaintenance than across the site as a whole.
What people tried
Every workaround mentioned in the threads below. We haven’t tested any of them — and nobody here is claiming they worked.
Found another way around this?
Share your steps and trade-offs so fellow engineers don't have to reinvent the wheel. No account required.
- 1covering the cup with cling film to test for dustDid this help you?
- 2cleaning cups with dental appliance or denture cleanerDid this help you?
- 3testing water qualityDid this help you?
- 4Asking online forums for identificationDid this help you?
- 5Checking for foul odors or active floodingDid this help you?
- 6dumping out the ice, washing the bin thoroughly, and cleaning with bleach waterDid this help you?
- 7Suggesting the debris might be coconut husk from carbon filtersDid this help you?
In their words
Unedited, grouped by where they were said, most upvoted first within each place, each linked to the thread it came from.
“I’m on well water and get my water tested regularly with no issues. This tiny film always appears if I leave water standing for about a day or more. What is it?”source ↗
“We have been caring for a family member's home that's been uninhabited for 6 months. Just found this in the ice cube tray of the freezer. Wondering what it is if anybody knows?”source ↗
“It is not hard and kind of disintegrates/melts between my fingers but does possibly feel a little oily.”source ↗
“i have a similar issue but mine is brown. eww”source ↗
“I noticed the water looked a little murky. Seems to be powdery black sediment at the bottom.”source ↗
“I lifted the float to test it was still working and noticed some gunk on it.”source ↗
“Do I need to be worried about the murkiness and gunk, or is this just how things are with the plastic basins?”source ↗
“I just noticed this in the drain and am kinda grossed out.”source ↗
“I just wanted to make this isn’t something I should be worried about affecting the furnace or water heater (or me).”source ↗
“So im a plumber by trade, and installed a water softener in my home with a spindown filter, im just curious as to what this is that is collecting in it, its hard and doesnt dissolve when rubbed, well is about 25 years old”source ↗