I have about 10 self-watering containers on my balcony garden. In taking care of them, I’ve started to notice that the reservoir containers have been starting to stank and form algae.
The reason for this might be because there is no room for the air to circulate. The air can get in through the overflow hole, but no other way to get out.
What I’d recommend is to add 2-3 additional drainage holes around the reservoir container to allow the air to freely circulate.