Oranda fish turning black?

I have had an orange oranda near black striped fins and tails for about 1.5 months now. There be two but, both had a bad case of white spot and the one that died have severe fin rot as well ( the dead fish also had these 'black patches'). I hold been treating the tank and the white spot has cleared up fully. Now my fish enjoy got black patches on his body and a few on the fins. It started of small and has grown more over the week or so it begin. I have been changing the sea 25-30% every 3-4 days ( recommended by a fish shop because of the white spot treatment). I have now only be changing the water 25-30% every week.
The fish doesn't look unhappy, he is still ingestion and swimming.
I got my water tested when the old fish died and adjectives the levels were about right expect the nitrate/nitrite ( achieve confused between the two?) - the one to do with high waste/food level. I enjoy been cutting down the food and following directions from the fish shop.
I'm not an experienced fish keeper and a short time ago want some help on my fishes problem?
its jus changing its color maybe to blend contained by the surrondings
my goldfish turn black because i put black rocks
How should I festoon my fish reservoir?
The Orandas have this problem so plentiful times. Sometimes the sellers apply dyes on the fish to color them, and these dyes start going away after a while. Also check the Ammonia level and the pH height of the water in the tank. Also try to make over the feed.
I want to fine-tuning my gravel surrounded...
Answers:    Sounds like ammonia burns. Test your ammonia, any reading other than 0 is a problem and will be poisoning the fish. What size cistern is it? Goldfish need (surprisingly) large overfiltered tanks, more than what they gain marketed for needing, far more! Do bi or tri weekly water change of at least 30% and keep a very close eye on that ammonia even - make sure your filter is installed properly and is not being overwhelmed. Only nurture every other day at the most and do not let any muck build up on the substrate.
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