Emperor tetra swimming sideways

[Leonor Filipe]

Hi,
I've had some emperor tetras for 5+ months now and a few months ago 2 got sick, they had severe buyoncy issues, one had a crooked spine and an infected injury as well (the other didn't). I did daily water changes and put medicine in the water suggested at my local fish shop and luckily none of the other fish died (one did seem to be having buyoncy issues but she got better). This was a terrible ordeal and we tried to understand what happened but all the levels were good and the water and filter changes had been regular.
I'm posting because a couple of my tetras are showing buyoncy issues again and one, a male, is particularly concerning. He keeps tilting sideways and is showing great effort to swim and stand upright, he was also found hiding behind the filter (a ritual seen in the fish that got sick and died). Again, all the levels are fine and the only "new thing" in the tank is a snail I purchased at said fish shop.
I'm concerned whatever issue happened last time is happening again and I really don't want any more casualties.
I'm very concerned and was hoping someone could help me figure out what's going on.
PS- whatever was happening before it seemed to be affecting the fish in order of size, the smallest were affected first (considering those who died and the ones that got better). This time it's following the same pattern.

 

[Colin_T]

Any pictures and video of the fish?

Have you tested the water for ammonia, nitrite, nitrate and pH?

What are you feeding the fish and how often do you feed them?

How often do you do water changes and how much do you change?
Do you dechlorinate the new water before adding it to the tank?

What sort of filter is on the tank?
How often and how do you clean the filter?

 

[Leonor Filipe]

Any pictures and video of the fish?

Have you tested the water for ammonia, nitrite, nitrate and pH?

What are you feeding the fish and how often do you feed them?

How often do you do water changes and how much do you change?
Do you dechlorinate the new water before adding it to the tank?

What sort of filter is on the tank?
How often and how do you clean the filter?

Any pictures and video of the fish?

Have you tested the water for ammonia, nitrite, nitrate and pH?

What are you feeding the fish and how often do you feed them?

How often do you do water changes and how much do you change?
Do you dechlorinate the new water before adding it to the tank?

What sort of filter is on the tank?
How often and how do you clean the filter?

Ammonia, Nitrate, nitrite and ph are all within the idea parameters.. I've been doing about 35% water change weekly and I always put the dechlorinate product before I add it. I use the tecatlantis bio boxes (charcoal, aquaclay, glass rings and coarse foam) and I clean the filter every couple of weeks.. The fish are fed daily some micro pellets and I also add some large sunk pellets for the Cory's.
I've tried to take a picture or a video but he keeps hiding and I can't get anything. I'll post a photo when he comes out.
Is there something I'm doing wrong? I did another water change yesterday (second one this week) cuz I'm concerned he's sick and I'm thinking on not feeding them for a couple of days in case this is an overfeeding issue.

 

[Colin_T]

Fish don't get sick from eating too much. If the fish eats, feed it well so it has lots of nutrients to help it fight off whatever this is.

 

[Leonor Filipe]

Thanks, I'll do that.
I have my lights on a timer and now that they're off he's not hiding as much, I was able to get these 3 pics but not much else, sometimes he's only tilting as seen in the two, but most of the time that just seems like too much of an effort and he let's himself go sideways as seen on the third.
I'm really concerned.
I'm also considering this might be stress related since we have a male emperor that has been showing some aggression lately and has been harrasing the other tetras. What do you think?

 

[Colin_T]

need a video to see the odd behaviour, cant tell anything from the pictures