Is this fin rot?

[rileyp]

Hi all my 10 year old son has diagnosed our goldfish with fin rot.
Heres a pic. We are setting up another tank now to put it in.
Is it hard to treat and how long will it take?
Any tips much appreciated!
http://img821.imageshack.us/img821/1307/screenshot046.png (http://img821.imageshack.us/i/screenshot046.png/)

Uploaded with ImageShack.us (http://imageshack.us)

[jimmy]

hi what are your water conditions plz ph ect

[rileyp]

tank has been running for about a month now
ph is 7
hardness is high at 250 need to lower this...I havent bought anything yet...to do this. suggestions?
nitrite appears a s 0 ppm
nitrate is about 20 ppm
alkalinity was 120
There are 4 old fish in a 360 litre tank...
I'm just trying to get conditions right in the tank and then we will replace the goldfish with guppies...
thanks Peter

[jimmy]

normally fin rot is from ppoor water conditions did you cycle the tank first before you put fish in if not i would change 3o percent of water everyday until nitrate comes down get from lfs a bottle of seachem stress guard this will promote healing:)