Urgent..Help!!....Black Patches on Gold Fish

Question
I have observed during last 30 days that of the 2 gold fishes i have in my 55 gallon tank, only one has developed wide black patches on both sides of its belly, tail, mouth & eyes. Worse news is that it is only increasing each day.
The tank was set up in June'07 & has undergone cycling. I do weekly water changes and vacuuming.But despite that the nitrate levels reach high levels (25mg/L).
The fish retailer tells me i need to change water and do vacuuming once a month, but in my case even though i go for 50% weekly water changes(sometimes once every 4-5 days) still nitrate levels reach rock high((25mg/L).
Kindly advise & help!!!.
Currently my readings are:
PH: 7.5
Ammonia : 0 mg/L
nitrite : <0.3 mg/L
nitrate : 25 mg/L
(Kit Name: Tetra Test Kit.)
Water Temp: 28 deg. Cel.

I have two Eheim filters in the tank, their gph is unknown.Should be able to circulate water in tank once every 10-15 mins.

<a href="http://img126.imagevenue.com/img.php?image=47178_IMG_2606_122_995lo.JPG" target=_blank><img src="http://img126.imagevenue.com/loc995/th_47178_IMG_2606_122_995lo.JPG" border="0"></a><a href="http://img22.imagevenue.com/img.php?image=47191_IMG_2611_122_1083lo.JPG" target=_blank><img src="http://img22.imagevenue.com/loc1083/th_47191_IMG_2611_122_1083lo.JPG" border="0"></a><a href="http://img213.imagevenue.com/img.php?image=47198_IMG_2613_122_8lo.JPG" target=_blank><img src="http://img213.imagevenue.com/loc8/th_47198_IMG_2613_122_8lo.JPG" border="0"></a>  

Answer
Hi Jagpreet Walia: Thank you for the pictures.  I have studied them closely.  I have to confess that I have never heard of a black fungus or any other such ailment.  I did research this a great deal and came up with nothing.  The pictures remind me of color pigment change in your fish and not a fungus or parasitical disease.  I would not treat the fish until it starts to show other symptoms of a disease as I am almost certain that this is just a color change and not a disease.  Your nitrate levels are up just a wee bit and I am going to attribute that to the large percentage of water that you change.  Gold fish are dirty fish... messy! Change only 20% of the water on a weekly basis or you will destroy the bacterial balance of your tank which would cause the chemicals levels to elevate.  Try that and see if that helps...give it about 1-2 weeks... if the chemical levels do not self adjust... let me know... dave