His tail was bitten off around six months ago but he was fine up until about 3 months ago. I know I was overstocked and I've changed this.
About 3 months ago he struggled to swim to the bottom. He floats at the top. Had this before a long long time ago with another goldfish and cured it by changing their food. But this one isn't getting any better.
I've tried peas, all the medicines they stock at my local pet shop but nothing is helping. I've noticed over the last fortnight his sides are becoming pink/red. It looks painful. I've uploaded a picture.
Is euthanasia the best option? He been in his own tank for the 3 months since getting ill. For about a week now he's been unable/not had the energy to swim to get the food I put in.