If it looks like blood under the skin, it is septicemia.
Septicemia can by caused by a variety of things, it is often caused by water quality problems or general stressful conditions in the tank environment, which then lead to an internal bacterial problem- it is lethal if left untreated.
You need to look at the ways you are keeping your fish to find the root cause of the problem if you are to save your fish. Because you are a new to fishkeeping, i would strongly advice reading this article on understanding water quality;
http
/www.fishforums.net/index.php?showtopic=10099
If you just filled up the tank, stuck some goldfish in it, and left it at that, then it is likely that your tank will be experiencing very bad or unstable water quality at the moment which will be putting a lot of strain on the goldfishes health and making them stressed.
If you can keep good water quality and understand how it works (like the nitrogen cycle), then that is the first step to keeping happy and healthy fish.
I need more info on your situation though if i am to help you sort this problem out, so;
What are the dimensions of the tank i.e. width, length and height?
Is the tank filtered, and if so, what type of filtration do you have and how many gallons/litres does it claim to filter up to?
Do you have any dechlorinator/water conditioner (to remove chlorine and heavy metals from tap water)?
What food do you have for the goldfish and how much do you feed them and how often on average?
Keeping goldfish is a big commitment, they can easily live to over 20years old if properly cared for, and many varities can grow very large (over 12inches long). Although they're often as tough as old boots, they need to be cared for properly like any other fish.
