I have 4 small fish in my aquarium (two Zebra Danios and 2 others which are a similar size but I don't know what they are). Every few days a lot of small bubbles appear at the top of the aquarium. When removed they reappear within 2 days.
The aquarium is a 15-20 litre one. It is cleaned every few months and has no filter. They are usually fed the right amount.
The bubbles were there for a few days and a thin dark layer formed on top:
A photo from the bottom view of the tank:
The water below is clean.
Is this harming the fish? What can be done to prevent this?
Any help is appreciated.