Something is slowly killing off my fish

[Gretal]

Approx 200litre coldwater tank, with filter, air curtain thingy. No real plants (plastic one's), and a large piece of driftwood that is still staining the water.

My goldfish only seem to live a few months and then they die. They tend to sit on the bottom (although one was doing nose stands), before having problems keeping upright, and laying on their sides. Then they usually go belly up within a day or two.

I'm pretty sure its not my water, as it is only one fish at a time, and usually a few months after I buy it. Up until then, they look fine. I have one at the moment who is looking abit odd, sitting on the bottom, sometimes gulping at nothing (as if its swallowing food but nothing is there)

For years I had goldfish that had the bare minimum of care and they thrived. Now I'm actually trying to look after them, and they're dying

The one's I've had lately have all come from the same local place. They are bred in big concrete ponds/tanks outside, sometimes the water so dirty you can't see whats in there. Is there a chance they are just a dodgy batch of goldfish? Or is there something I'm missing?

I would've thought if there was anything major going on in the tank, they'd all die together, not one every few weeks or so. (dependant on when I buy them, they seem to last a few months.)

Am feeding them flakes, but they do get pee's a few times a week.

Any help please? I don't want to continue having a goldfish graveyard

[Fee]

Have your water tested, plenty of water quality issues can cause long term problems, usually resulting in lowered immunity which increases the prevelance of opportunistic disease or general malaise.

Poor water quality can often result in newly introduced fish doing poorly whilst fish that have grown accustomed to poor water may "appear" fine, but will have a much decreased life expectancy.