Water cooler and ....bugs...

So we bought a bench size water cooler, mainly as we were running out of fridge space with all the bottles of cold water we had in the fridge.
We have had it 2 weeks. It's a 12L bottle, it lasted a week, and I then refilled the bottle from the tap, as our tap water is really fine, we just like it cold. Anyway, it is now about half way empty and there are bugs inside it!! Little black fly like things....size of fruitflys but not...
WTF!
Where, how, why?? Anyone else had this? I cleaned it out before we turned it on, and like a said 2 weeks, surely it's not that bad!
Help please!!

Like underneath the filter part? Or the lid?
We have had ours on for a really long time and never ever had that.

In the bottle, its one of those big 12l bottle things you turn upside down. The bugs were floating in the bottle

I have one of those and gave never had a problem with bugs. I wonder if they were already in your tap water?

Hmm...in our tap. Do you think that is more the case of them already being in the water, than getting into the system after I'd filled it?

All I can find on Google is recommended cleaning ever month or two...but they all say to use bleach....I'm a bit hessitant to use bleach in our drinking water.... Do you think vinager will cut it? Or just bleach it? What does everyone else do?

I've never even thought about cleaning mine. I'd probably use bleach if it was slimy or smelly inside but make sure it's rinsed well.

I just did a google search and a few people have had bugs appear in their water. They don't say how they got there, just how to clean it out.

Weird!!

I just use a stiff brush and dishwashing liquid and rinse well. Mine can also go in the dishwasher. There's already chlorine in the water to reduce bugs so washing a vessel in it isn't really a big deal if it's rinsed well. I use it for our aviary birds.