I am new to keeping goldfish in a tank - I have kept goldfish in an outdoor pond successfully for 2 years (having only lost one fish)

We have a UK 35 litre tank that has been set up for almost 2 weeks with a air pump and trickle filter and have 2 small fantail goldfish (called Nemo and Dory). Dory is mostly gold with the tips of the tail being white, Nemo is the smaller one of the two of them and when we got him was mostly gold coloured but the tips of his tail and fins were black.

I took water readings yesterday and are as follows:

    Ammonia....0.....ppm
    Nitrite........0.5...ppm
    Nitrate.......50....ppm
    pH............7.5....
    GH...........250
    KH...........100
    Temp........haven't check but it is a cold water aquarium

I have carried out a 50% waterchange 9 days after setting up and 25% change today

I am using a Tetra Aquasafe conditioner which says to use 5mil per 10 litres of water

Dory is perfectly healthy and eating well, swimming around all the time.  Nemo on the other hand started out very active  but then suddenly started becoming very lethargic and swimming differently. He is going and hiding in his little cave (tucking himself right in to the smallest part). He stays in this cave for hours and doesn't move, then he will pop out for a little while before going straight back in to his cave. Now he is starting to get black across the top of his head and it still seems to be spreading, his goldness seems to be fading too as he was quite a deep orange when we first got him.  Up until yesterday he seemed to be eating but even though I have been careful to put food directly where he has been swimming today and watched him when the food has been near him he doesn't seem to have eaten anything.  

I've just checked now and he is back in his cave and I can see his gills moving so I know he is alive but he not moving at all, just the bear minimum.  He's been like this for 5 days now.  He was very active when we first got him so cannot understand why he has suddenly gone downhill so quickly.  Dory is still very active and eating loads (in fact she is now eating Nemo's share too!!).  Its mostly impossible to take a picture of Nemo now he is sick, as soon as he comes out and I go and get the camera he disappears back into his little cave but have managed to get some today!

I did ask some advice at my local aquatic centre where they said to try changing the water and I got the water testing sticks.  They also sold me some chemical to put in to help diseased fish. This was put in on 13th July when I did the water change.

I think he seems to be getting worse athough we thought on 12th July that he was already dead (twice).

I don't want to loose him so any advice would be greatfully received.
 

Thanks for coming back to me.  Unfortunately Nemo has now passed away - he was alive at 9.00am this morning but by 3.30 this afternoon he had gone.  I will carry out the water change, however Dory is swimming around still perfectly healthy!

I will have some words with my aquatic centre where I brought the tank from as they told me that I could keep up top 5 fish in this tank!  Originally we brought the tank for 4 baby goldfish that we found when cleaning out the garden pond but they all died one by one! After 2 weeks they had all died.  I had totally cleaned the tank and gravel out and put Aquasafe in.

I will go and carry out a water change now anyway and try and get the nitrate levels down.

Thanks

I bought a 90L tank  on-line(UK supplier) complete with heater, internal pump/aerator/filter/hood with lighting for under £100 inc delivery....I keep two baby Ryukin in it...I consider this to be the absolute minimum for 2 fancy goldfish.