Fry acting strange

I did a 20% water change on this tank this morning and noticed an hour or two later that this one sawbwa fry was acting strange. I decided to do another 20% water change a few hours later but nothing changed. All the other fry are well and eating except this one. It also has moments where its body goes still and just floats around in the flow and then suddenly starts swimming again. I've had this happen to adult fish over the years but not fry. Any ideas about what's causing it?

Never mind it died I took it out for closer inspection its has a red area on its stomach. My guess is a puncture

If it happened straight after a water change, it's possibly chlorine/ chloramine poisoning.

Did you dechlorinate the new water before adding it to the tank?

Water companies sometimes increase the chlorine/ chloramine levels if they do work on the pipes, if there's a damaged pipe, or when it's really hot. They don't tell you about it and people regularly lose fish from this.

Colin_T said:

If it happened straight after a water change, it's possibly chlorine/ chloramine poisoning.

Did you dechlorinate the new water before adding it to the tank?

Water companies sometimes increase the chlorine/ chloramine levels if they do work on the pipes, if there's a damaged pipe, or when it's really hot. They don't tell you about it and people regularly lose fish from this.

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I always use tap water but dechlorinate it before use usually leaving it 10-20 mins standing. I use the Fluval water conditioner. I also usually add excess doses since I have a water tote and don’t really measure amounts. Maybe I didn’t dose enough with that first water change? Perhaps you are correct about the possible increase in chlorine as I had noticed an increased haze to the water afterwards which I suspected to be debris in the water column but it may have been chlorine. Thanks Colin for the help though

The split in the skin could be an indication of internal problems. Do any of the others have the pink patch on the chest?

Just had a look there is potentially one other that has pink-ish colour on the chest. The behavior of that fish is normal. The one that did die was smaller than the others. I have about 4-5 of the fry that are quite smaller than the others.

Also on the topic of fry is it normal for there to be somewhat sparring matches? These fish are getting to about 4 months old now and I've got a few with nipped fins already.

Young male fish (teenagers) will often spar and show off to each other.