I introduced 2 small fantail goldfish to my new 6 gallon (UK gallon) pretreated tank about 2 weeks ago. I have done a partial water change (about 25-30%) once so far.

I noticed small white patches/spots on its tail last week, which have now gone but have been replaced by grey/black patches (not spots) on the base of his tail, and one side of his belly. Also, he sometimes flicks in the water suddenly. Just before getting the grey patches (when the white spots were disappearing) he was acting very lethargic and floating at the top of the tank. The other fish is fine with no visible patches and I have also tested the water several times and all levels were normal (nitrate, ph etc).

Is this some kind of fungus?

Also, I have some Mopani in there which developed (before I put this fish in) white growths/spots on it which the fish now like eating, could this be contributing to the problem? Should I have soaked it in hot water before putting it in the tank?

Thanks in advance

Okay first of all, regular comet goldfish need 20+ gallons and fancy goldfish (like yours) need at least 10+ gallons each . Also the white spots earlier were and still could be cases of ich, a common fish ailment. Why they turned black/gray, I don't know. You could remedy it with a little salt in your water, but I'm not experianced in telling you what you really should do, so you should wait for some others to repond.

P.S. Copy and paste all the questions above and then answer them all too, we may be able to help even more.

Hello there and sorry to hear you're having problems.
Wow! 2 fish in a 6 gal is trouble right there. You really need a minimum of 16 UK gals at very least.
Additionally, in a new tank there is no beneficial bacteria to process the fish waste (ammonia) so with only 1 tiny water change in 2 weeks your water will be pretty toxic. Unless of course you have pre-cycled it or seeded the filter from another tank? Water probelms in a new tank can cause all manner of illness so we never try to diagnose until we have the water stats.

So before we go any further, can you post back with the answers to these;

Please provide the following details when requesting help for Goldfish Problems: Test Results for the Following:
Ammonia Level?
Nitrite Level?
Nitrate level?
Ph Level, (If possible,KH and GH and chloramines)?
Ph Level out of the Tap?
Tank size (How many Gals) and How long has it been running?
What is the name and size of the filter/s?
How often do you change the water and how much?
How many fish in the tank and their size?
What kind of water additives or conditioners?
Any medications added to the tank?
Add any new fish to the tank?
What do you feed your fish?
Any unusual findings on the fish such as "grains of salt",
bloody streaks, frayed fins or fungus?
Any unusual behavior like staying
at the bottom, not eating, ect..?

You can copy and paste this onto your new answer.

Things to do right away:

Reserve some water for testing.
Do a 100% water change to relieve them of toxicity - temperature matched and de-chlorinated.
Also, take the time to click on the two links below my signature, to make sure you understand the basics.

Hopefully we'll have your fish looking better soon. But you must address the problem of space. In this small tank you will not be able to keep your fish healthy.
Post back when you can

The tank was declorinated, and I added bottled bacteria before adding any fish to the tank, and ran it for a week.

The readings just before I did a (25%) water change were:

GH: 180
KH: 240
PH: 7.5
NO2: 0.7
NO3: 10

Tank change: 20-25% every 10 days.
Addititves: Filter Start Dechlorinator, and Filter Start Aquarium Bacteria

No new fish, no medications, no other unusual findings etc.
Feed them flakes, and then mashed peas a couple of times a week.

I should clarify that the white spots did not turn into black ones, they infact went away completely (and therefore may be irrelevant). These black/grey patches are in new places on the fish and are slowly continuing to spread. The other fish is still fine.

I think the numbers above suggest the tank is ok chemical-wise so I dont think its ammonia burn.
Does this sound like some kind of fungus?

I don't see a reading for ammonia?

Thing is even using a cycle product it is impossible to cycle a tank in a week. The fungus is a reaction to poor water, the black most likely ammonia burn.

If you read the links under either Trinkets signature, or mine, you will learn more about the importance of water chemistry.
What you have going on is called 'New Tank Syndrome'. When a tank is new you must change water every day to keep the toxic chemicals from rising above safe levels. 20-25% every 10 days is dangerously little.
Ammonia and nitrite need to be kept as close to zero as possible - your nitrite is 0.70, so needs to be lowered. It will damage the gills and hinder the uptake of oxygen.
Eventually you will grow enough beneficial bacteria to process your fishes waste but at the moment there is not enough to do the job.
If you have an ammonia tester please run it asap - if not, you will need to buy one in order to monitor what could be very dangerous levels. As Trinket pointed out you have no reading for ammonia but the symptoms tell us that the level is likely to be pretty high.

Do a large temp-matched water change as soon as possible - and from now on, test you water daily. Be prepared to change water daily, according to the results. If ammonia or nitrite climb past 0.50 you must do a partial change.

Do post back and let us know how things are progressing.

Welcome to Koko's