I've had my 28l aquarium up and running for about 6 weeks. We've battled with new tank syndrome and un fortunately lost an Oranda and Black Moor to a mysterious illness which started with Ick.
I treated the water with whitespot treatment and changed 75% of the water and left for a week before introducing a gold Teloscope eye and a Nymph. They both seemed very healthy and looked as though they were flourishing, however since changing 20% of the water a week ago the water chemistry has gone bonkers. Nitrite levels increased to 3mg and the water went quite milky. This sent Oskar the Telescope into what can only be described as a depression. He lies at the bottom of the tank with dorsal fin clamped down and tail fins closed. I noticed him thrashing around at intervals and he had a few white spots so I treated him with White spot control (2 treatments so far with 48hr intervals). The spots seem to have gone now but he seems to have a slight pinecone effect on the scales down either side of his back/ribcage. He still seems to be sitting at the bottom and thrashing occasionally. Elmo the Nymph also seems to be thrashing around and rubbing himself against objects but he has not got any other visable signs other than his dorsal fin clamped down.

I have addressed the water chemistry with frequent water changes and have just started to treat the water with Amquel plus, for the nitrite's and amonia levels, aswell as P-Clear particulate to clump the loose milky particles for easier filtration. The nitrite levels have now fallen to less than 3mg and the water is clearer. I have also started to treat both fish with Myxazin aswell as whitespot control. Both fish seem to be eating (although the telescope's appetite seems to have diminished slightly) and they seem to be pooping alright.

Please help me, should I be doing anything else? Being a newby to the joys of Goldfish keeping I'm scared in case I lose another 2 of my daughters 1st Birthday fishies.

Do you have aquarium salt in your tank if you do you must do an immediate major water change as salt will not help in cases of dropsy as the fish's body will absorb the salt.It would also be wise to quarantine your telescope.

Hi nudge,and welcome.
28L's is just a bit over 5 US gallons and not enough room for one Goldfish let alone more.
Oskar the Telescope isn't depressed,he's showing signs of sickness because the water quality is so poor.
the water chemistry has gone bonkers because you've introduced 2 new fish at the same time,your tank is too small,and apparently your Bio-load wasn't able to handle that much Ammonia/waste etc.
Nitrite levels need to be brought down immediately,by large water changes with treated and matched temps.
Thrashing,fin clamping etc. could also be a sign of Parasites,but we can't know that until your params are in control because toxic water will give your fish the same symptoms. Treating with meds especially not knowing what the exact problem is for sure can only make things worse,especially when your params are so off.
Your one fish now sounds like he's showing signs of dropsy,and it's more important than ever to continue water changes until you get those levels under control.My best advice to you would be large water changes,and another larger size tank,or even some sort of Rubbermaid container.as long as you have these fish in a small tank,you'll continue to have these sort of probs.It's really hard to advise you on where to go next ,with that fish with Dropsy because that is a very serious symptom that things are not right.Sometimes getting the Params right will fix it,but alot of times,by then it's gone bacterial and needs more advanced help.As I mentioned earlier,if your params are bad,then using meds is almost a complete waste.

Where are you right now with things?Have you done another water change?What are your readings?

Thank you Blue and Devs for your advice. The bad news is that Oscar the Telescope didn't survive the water change and quarantine. The Good news is that Elmo the Nymph has shaken off the touch of whitespot he had and is as perky as ever. I don't think he misses Oscar now so much, I think he did at the beginning as he would sit in the corner of the tank looking at his reflection in the mirrored back panel, not now though he is darting all around the tank. I seem to have the water chemistry at the correct level now and I don't intend on introducing any other fish until we get a larger tank. I still can't belive the twerps at the pet shop told us the 28l tank would keep 2-3 fish quite healthy. Thank you once again on behalf of my one year old daughter who adores her fishy friends, and myself who quite likes them aswell!!