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:

bottom view

The water below is clean.

Is this harming the fish? What can be done to prevent this?

Any help is appreciated.

  • 4
    you really need a pump and filter in your tank so you get the water properly oxygenated. Dec 16, 2018 at 18:31
  • Where do the bubbles come from? Are they sat in the gravel and then rise to the surface?
    – Henders
    Dec 19, 2018 at 9:15
  • @Henders I have found a solution
    – user13836
    Dec 19, 2018 at 10:39
  • Sure, but the questions and answers here are to help other people in the future primarily so it would be good to know.
    – Henders
    Dec 19, 2018 at 10:41
  • @Henders I don't see where they are coming from
    – user13836
    Dec 19, 2018 at 10:42

2 Answers 2


Fish take in oxygen through their gills and 'exhale' CO2 just like every other animal, so they may just be CO2 bubbles from their respiration. If those are real plants in the picture, it may be O2 bubbles from the plants.

You really should have a small filter in there. The danios won't mind, they like current. There are lots of small internal filters you can get for tanks of that size. In addition to filtering the water and keeping ammonia down, as Trond mentions in the comments, it will agitate the surface, which better oxygenates the water.

Also danios are fast swimmers, so 20L (around 5 gallons) is pretty small for them.

  • The plants are fake
    – user13836
    Dec 19, 2018 at 7:38
  • @DManokhin thats what I thought, but I couldn't quite tell from the photo. Dec 19, 2018 at 7:41

After changing the water I realised what the problem was. It was over feeding.

The bubbles rise to the top and don’t pop because there is too much protein present in the tank.

Starting to feed them less now and is far it is fine.

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