Sudden Danio aggression

[fishnan78]

Hi, my son is 14 and I am supporting him in taking care of his fish. He has 7 zebra danios, 14 neon tetras and 4 sucker fish (Can't remember the name of them) in a 126L tank. All have been getting along fine until this morning we've noticed that 2 of the smaller danios have become extremely aggressive towards 2 of the larger danios, chasing them relentlessly as though they're trying to pin them down. The one went limp this afternoon as one of them pushed it into the glass.

I don't have a tank divider or spare tank but I think I need to do something fast as the 2 being targeted must be very stressed. But I don't know what to do?

I've read that the larger ones may be pregnant females ready to lay eggs? But even if this is natural behaviour I can't see how the two "victims" can tolerate this for much longer as its been going on all day.

Any advice would be greatly appreciated.

 

[Colin_T]

It could be breeding.

Any chance of a video of the fish?
Upload videos to YouTube, then copy & paste the link here.
If you use a mobile phone to film the fish, hold the phone horizontally (landscape mode) so the footage fills the entire screen and doesn't have black bars on either end.

If they are all short fin danios the females should be fine getting chased by the males.
If you have short fin males and long fin females the females will struggle to stay away from the males.
If you have long fin males and short fin females they will be fine.

If they are breeding the fish will scatter the eggs among fine leaf plants like Java Moss and that will be the end of breeding for the day and usually for a few days to a week.

 

[fishnan78]

Thank you for replying. They've settled down now thankfully, hoping it stays that way! The fish that were being chased are quite round so maybe they are carrying eggs. I'm not sure if they're short or long finned or a mix, they were all purchases together so I expected them to be the same but some are quite a bit bigger than others and I'm not sure why.

I've included a video but the aggressive chasing seems to have settled.

 

[Colin_T]

Short fin danios have normal size fins and tail. Long fin danios have longer fins and a longer tail. It's quite noticeable if you see the 2 types together. If all the danios have the same length fins they are fine.

Your tank has too much water movement for the tetras. The fish are being moved around by the current in the water and they need calm areas to rest rather than being forced to swim into currents all the time. Turning the air pump off at night and for a few hours during the day might reduce it enough. Otherwise you might need to change the position the filter's spray bar to reduce the current. You might be able to reduce the flow on the filter too.

The blue and green gravel and aquarium backing (picture) is washing out the colours of the neon tetras. If you had a black backing it would make the neons much more noticeable. A darker substrate (brown or black) would also help their colours show up more. Some floating plants would also bring out their colours more. Water Sprite (Ceratopteris thalictroides/ cornuta) is a good floating plant and if it grows too much, you can plant it in the aquarium gravel where it turns into a lovely light green shrub.