Water barrel mold?

[mudd32]

I have a blue 25 gallon water barrel with the watering cups.
I'm getting back residue inside the barrel on the walls....assuming it's some type of mold.
I always put some apple cider vinegar in it when refilling and it has a sealed lid.
It sets in the shade the biggest part of the day.
I recall reading something a while back that if you paint your water barrel black it solves the issue.
Anyone know if that is fact or any other ideas to eliminate the problem?

 

[nuthatched]

Try barley straw bags.
Skip the acv.

 

[mudd32]

Try barley straw bags.
Skip the acv.

any idea how long 1 bag lasts

 

[nuthatched]

any idea how long 1 bag lasts

My coworker uses them in her goat tank, she says a few months, they're only $7 for two.

 

[mudd32]

My coworker uses them in her goat tank, she says a few months, they're only $7 for two.

ok thanks...we'll give it a try.

 

[swamphiker]

Reducing sunlight will reduce algae growth, which is why painting the barrel black would likely help.