Giant Danio problems

[Perrya11285]

I have 5giant danios and today one is swimming very erratically. It is floating all over the place while facing all different directions. There are no outward signs of disease.
It is an established 55 gallon aquarium that has been sent up for 11 months now. The tank is slightly overstocked because I am going to move giant Danios to new tank once I sent it up.
My ammonia and nitrites are 0 my nitrates are between 35-40 ppm as I am due for a water change. My pH has been consistently around 7.5. My water is quite hard at 12kH.
I have had no problems with other fish.
They haven’t been in the tank very long and when I did put them in I used drip acclimation.
Thanks for your help.

 

[V227]

Your Danio might have swim bladder disease. Usually no fixing that problem once the fish exhibits the noted symptoms.

 

[Autumnsky]

How long since the last pwc?

I would do 2 back to back 30% pwc if it has been longer than 2 + weeks.

If you always do weekly pwc then I would do a 50% pwc and make sure you use adequate amount of dechlorinator.

Pwc is my first line of defense after checking things like water temp, tank parameters.

Have found a bad heater before in a tank causing the fish to behave oddly. (In addition to pretty much every other problem known to fish keeping at one time or another!)