Harlequin rasboras suddenly ailing

[Catfish586]

I've got a tank with cardinal tetras, black neon tetras and harlequin rasboras. One of the harlequins died earlier this week: I just found him floating. Then tonight there's another clearly ailing - it's languishing on the bottom, struggling to swim (upside down at times) and seems to be gasping. It doesn't look like he'll be with us much longer. Anyone know what it could be and why only the rasboras seem to be affected? I'll check all the nitrate, ammonia etc levels and do a sizeable water change in the morning, but it looks like I'm about to be two fish down. Help?!

 

[Colin_T]

Any chance of a picture and short 60 second video of the fish?
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Wipe the inside of the glass down with a clean fish sponge.

Do a 75% water change and gravel clean the substrate every day for a week or until we work out what is wrong. The water changes and gravel cleaning will reduce the number of disease organisms in the water and provide a cleaner environment for the fish to recover in. It also removes a lot of the gunk and this means any medication can work on treating the fish instead of being wasted killing the pathogens in the gunk.
Make sure any new water is free of chlorine/ chloramine before it is added to the tank.

Clean the filter if it hasn't been done in the last 2 weeks. However, if the filter is less than 6 weeks old, do not clean it. Wash the filter materials/ media in a bucket of tank water and re-use them. Tip the bucket of dirty water on the garden/ lawn. Cleaning the filter means less gunk and cleaner water with fewer pathogens.

Increase surface turbulence/ aeration to increase the oxygen in the water.

 

[Catfish586]

thanks for this Colin. I'm afraid the fish has died as I feared. I did a water reading and the nitrates were on the high side. I've changed about 50% of it and will do the same again tomorrow. The readings are now OK. It may be that as the smaller fish the harlequins were most vulnerable to any less than ideal conditions. I've also removed algae, changed the carbon filters and rinsed the foam filter in the old water. Hoping that all this will help but all monitor closely.