Goldfish appears to be seriously ill

Hi

I have 2 goldfish, one about 8 inches nose to tail and one about 6 inches.
They are approximately 8 years old. They are kept in a 140 litre tank with
an ehiem external filter, airstone & heater (about 22 celcius). Nitrates,
Nitrites and Amonia are all below measurable levels. The smaller of the 2 is
looking distinctly unwell - it is lying on its side on the the bottom with
its nose in the gravel and arched so it's tail is also on the bottom. When
disturbed it swims away and it is taking food pellets from the surface. It
does appear to have lost a few scales recently, also its scales are standing
on end - a bit like a porcupine. The other fish is OK. Grateful for any
advice.

Regards

Neil