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Foaming at top of guppy fry tank


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I've had this issue off and on for a while. Sometimes my tank will get bubbles at the top, even after a water change.

I recently did about a 20% water change to my planted 29 gallon tank after I noticed the bubbles, because I was worried that the cause might hurt the animals, but the bubbles didn't go away. It has 5 baby guppies (~2 1/2 months old) and 2 nerite snails. There has been some foam at the top off and on since I've added the snails. The tank is cycled and it was fishless for about 4 months before I added 3 nerite snails, then 3 more. Then about two months after, I added the babies. I now only have 2 of the snails. I also use easy green and the aquarium co-op carbon for algae, but skipped both for this water change because I thought that it might be contributing. I haven't used either for the past three weeks.

The water parameters a day before the water change were:

Iron: 0

Copper: 0

Nitrate: 25-50 ppm

Nitrite: 0

Chlorine: 0

GH: 75-150-300 (I couldn't really tell)

Total Alkalinity: 120-180 ppm

KH: 120-180 ppm

pH: 8-8.4

Today, the water is:

Iron: 0 ppm

Copper: 0 ppm

Nitrate: 25-50 ppm

Nitrite: 0 ppm

Chlorine: 0 ppm

GH: 150-300 ppm

Total Alkalinity: 120 ppm

KH: 120 ppm

pH: 7.8-8.4


Edited by Thundercracker
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On 10/30/2023 at 11:49 PM, TheSwissAquarist said:

I recently had the same problem with a new HOB. It could be industrial residue? I drained the tank and wiped it down with a sponge to get rid of all the bubbles on the sides. 

Might be, I have an HOB too, I'll clean the filter and the media to see if that works because that thing does get pretty dirty.

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Bubbles like that form for a multitude of reasons. Industrial residue is something I would not be terribly concerned with. Sometimes grease will come off a new motor but the grease they use is aquarium safe and generally hydrophobic. It usually forms an ‘oil slick’ on the surface before dispersing 


If I had to guess the bubbles are oxygen that gets trapped on the surface by the layer of viscous fatty acids. It’s a layer that forms naturally by certain biological processes. It’s been a long time since I took my water chemistry course so hopefully I used the right name for these processes 😅

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