Betta tear fin on sponge filter?

[Any Huit]

I bought a sponge filter from aquarium coop for my 10-gallon aquarium, to try to switch from carbon activated filters. But now it looks like the two female bettas that live in there have long tears down their fins!

To answer the obvious and very understandable question - I got two female bettas more than 7 months ago. They were living together in the same tank at the pet store, and they've lived in this 10g tank since then, without fighting or chasing. Bizarre! Anyways, I've never seen this kind of injury before which is what makes me think it's the new sponge filter.

I know nobody can answer my situation directly, but I was wondering if anyone else has ever seen their betta fish get fin tears after introducing a sponge filter? It doesnt look like fin rot, just one long line tearing the fin. 

Sincerely, a concerned betta fish keeper

[nabokovfan87]

On 7/13/2022 at 3:33 PM, Any Huit said:

Anyways, I've never seen this kind of injury before which is what makes me think it's the new sponge filter.

There's definitely some sponges I've had that can be pretty sharp / plastic type of material.  Some vendors have a very low porosity sponge, but the material itself is soft.  I have others that are a bit more robust and have the plastic type of feeling to them (not really dependent on PPI).  I would suggest soaking the sponge in a bucket, typically for 2-3 days and every day just "work" the sponge so it's not so stiff.  This should soften the material and get it used to being underwater.

If you can, please feel free to share photos of the setup / injuries.

Edited July 13, 2022 by nabokovfan87

[Any Huit]

Thank you! They're very curious fish and were keen to swim around the new sponge filter, and the aquarium coop filter is a bit firmer and pokey. I'm thinking I might try the Top Fin sponge filter, which looks a bit softer. 

Here's a pic of them - if you zoom in on the blue one you can see the tear on the caudal fin. the pink one also has a small tear, but it's not as noticeable, so I'll just share this picture of Mermista: