black/brown fin edges

Question
Thanks,for the advice, but dispite my best efforts i unfortunately lost the fish. i still have a small black moor left but am worried that this to may be in trouble and is not looking too great. I have been adding cycle to the water and doing regular water changes as well as cutting down the feeding. i went to my local aquarium shop where I bought the tank and fish for advice but the owner seems to have no clue what so ever and is some what difficult to communicate with and seems more intent on selling me  things and charging me to test my tank water, which i am not convinced that they know how to do themself.I have got my own test kit (eSha aqua quick test) and have tested the water myself and these are my reading, can you tell me the problems and recommend some solutions to them.
acidity is ph 8.4 or higher, the tap water where i live is hard and may be the cause of this problem. KH value for carbonate hardness is 13 dH, GH value or general hardness is >21 dH  nitrate 100 mg/l and nitrite 1 mg/l
Thanks
from Mark
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Followup To

Question -
I recently set up a tank, for my daughter and the fish had been doing fine, I have noticed that a fantail goldfish I have has developed a black/brown edge to some of it's fins, in particular the two small ones behind the tail.
Any Idea what could be causing this, is it serious and what is the treatment.
Thanks mark

Answer -
Hi Mark,
Thank you for your question. Here is my page for wholistic aquarium care:
http://steamboats.com/aquarium.html
Rotting fins tells me the water may be polluted. Please reduce feeding to one flake or pellet per square inch of fish per day. Too much food fouls the water and creates all sorts of health issues, and often results in death.
Treat the water with Cycle, a product you can get from the aquarium supply shop. It's friendly bacteria that will help purify the water.
Follow the instructions on the bottle and store in the fridge between use.
If the fish is in dire straits, follow the emergency instructions at the bottom of my page (above) and please print the page for future reference.
Write back if you want to discuss further. I'll be out of town for a few days.
Nori

Answer
Hi Mark,
Thanks for the follow up question. Sorry to hear about the other fish.
Now that the water is clear and you have the fish on a more reasonable diet, you can go back to changing the tank every two weeks. Just give the fish time to rest & heal.
Nori