Goldfish inactivity, possibly sick?

[Hendre]

Hi guys, one of my goldfish has started being unusually inactive.

This morning he was up and about with all the others but now he sits at the bottom of the tank without moving and when i get close he swims around like the others but not extremely well. All the other goldfish are fine and the unwell fish doesnt show any physical signs of illness after a thourough checkup.

I did a 50% WC 3 days ago and after testing now (250 litre tank, +- 70gal)

PH: 7.8
Nitrate: 15 Mg/L
Nitrite: 0
Ammonia: 0
Hardness: do not possess kits but am in a 'high' hardness area
Temp: 19 degrees Celsius, 66 Fahrenheit

Feeding floating pellets once daily and bloodworm once weekly

Any ideas?
My picture file was too big ;(

 

[CarpCharacin]

It would be really helpful if you could post a picture.

 

[Wailua Boy]

What variety of goldfish?

 

[Wailua Boy]

I would guess without knowing anything else about your system, a gastrointestinal issue as goldfish don't seem to flourish for long without a fair amount of vegetable matter in there diet. Hit him/her with some skinned peas and then add some water plants to feed on.

Put em' on the "pealax" diet

 

[Hendre]

Its a shebunkin and he is back to his normal self after some peas, probablt was gastro, thanks Wailua Boy

 

[Hendre]

Checked again now, they are all good and well, and as a plus the neighbours found my snake in their wood pile after his disappearance 2 months ago