Leaking tank

When I drilled a new well I put in a new tank and kept the old one. It's plumbed up so that it only feeds my outdoor faucets. It's an old green fiberglass tank about 6' tall, about 24" OD (80 gallons?), about ten years old. A breather, or whatever you call it, on top lets a slow drip escape. Not enough to cause any puddling, just green slime on side of tank. But some law of hydraulics tells me the leak won't get smaller, only bigger. I'll probably replace tank this summer. My question is: can I repair or replace bladder? If so, cost advantage? Or am I on the right track to replace tank? I'll definitely put in a smaller tank when I do.

Thanks.

Bill