I have introduced several news fancies into my 600l over a period of time, both jumbo's and tiddlers and never had any bullying issues - sure the little omnes can be rather intimidated but the bigguns don't bother them at all.
They need at least 2 weeks in a Q tank, gives you time to add salt / medications and time to notice any abnormal behaviours - just make sure you've got a filter with cycled media on the Q tank otherwise ammonia will go through the roof in the Q tank and will do more harm than good.
Test the water in the Q tank daily and perform regular water changes, you may need to to 50% every day for the 1st week.
When it comes to gel food I feed my fancies Repashy Soilent Green, can't praise it enough - although I still feed mine Hikari food on occasions their staple diet is now soilent green as I get no floaty issues with this at all, cheap too as I keep it all frozen so it lasts for months
The storage box would become his new tank permanently until he is well enough (and the weather is warm enough) for him to go into a pond, unless of course you are prepared to upgrade your tank to something much bigger but that could be space / cost prohibitive for you and despite all of that you still need to re-home him into a pond anyway.
Difficult to say what that lump could be, could be a food impaction / constipation as I mentioned earlier or could be something more serious like a tumor which you would be unable to cure really.
When fish are constipated they do tend to go off their food (and you mentioned his appetite had dropped).
I would withhold hold for 3 days and then feed a couple of peas using the method I mentioned earlier which should break down the impaction if that is the problem. If you are unable to separate him from the others they will all have to go without food for 3 days but they should be fine as goldfish can typically go for over a week without food.
In terms of a temporary tank you'd need to look at something that would hold at least 200l of water but bigger the better really.
A cheaper measure might be to buy one of those "really useful storage boxes", they can hold large amounts of water and are a low cost alternative to investing in a bigger tank if, long-term you don't actually want a bigger fish tank or don't have the space for one.
Similarly any kind of plastic, food-safe storage box that holds a large amount of water would be suitable for your comet, as long as it's well-filtered and you keep up with the usual water changes - keep him in there on his own though to ensure he is getting all that space to himself, then when the weather warms up in spring you can look to re-home him to a pond.
As ?bouncinsheep? mentioned earlier your comet needs a pond so a bigger tank isn?t really going to help.
Unfortunately your situation is what happens when fish who require larger environments in which to live are placed in small tanks that will basically end their lives prematurely, treat this is as a learning process.
This lump could possibly be food impaction (constipation) so it wouldn?t do you any harm to feed him a couple of peas as this may help relieve any constipation he may have, although of course this may not be the issue and I know you mentioned his appetite is poor.
Take a couple of peas from your freezer, put them in a cup, pour boiling water into the cup, I also put the cup into a microwave for about 1 min or so, then when the water has cooled take the peas out, remove the soft contents from the outer shell (this is important as the fish can?t digest the shell), squish the contents a bit and then feed to him, this may help.
If this does help, great but you definitely need to put him in a pond.
Ok it?s going to be really difficult for anyone to help unless you can test the water and let us know the results.
One thing I can tell you though is that your tank is very overstocked, believe it or not your tank is only big enough for 1 fancy goldfish and this may explain his health issues ? when your fish were smaller it may not have been such an issue but as they have grown over time naturally they get bigger and thus produce more waste and you are now in a position where you definitely need a bigger tank or must drastically reduce the fish population in your tank. It would also help to increase your water changes to 50% and perhaps do this twice a week, this may help you to reduce the toxicity of the water in the short-term.
Without test results or pics I am unable to help further but just wanted you to know that.
This won't be what you want to hear but I would never recommend anyone move a tank that has any water / gravel / fish in it - I tried it once years ago with a 200l tank, I reduced water down to a minimum and tried to move the tank with a friend. Believe me, even with the minimal water level the tank weighed an absolute mega-ton and we could only move it a couple of feet before giving up, we ended up stripping the tank down to move it and this ended up being the best course of action anyway as the fish were pretty stressed by just that 2 foot move we managed.
It is risky to move a tank with any amount of water / gravel in it as you run the risk of it cracking, it's not worth the worry or the risk of permanently ending your tank for good.
Even though you are only moving it a small distance, I would advise stripping the tank down to bare glass and moving it sensibly and then setting it back up again, it shouldn't take too long and will actually be less stressful for the fish really.
Do you have a container to keep the fish in while you move it? Your filter bacteria should survive provided you don't drain all the water out of the filter and the process doesn't take several hours (I am assuming you have an external filter but if you don't it would be even easier with an internal as you can just put that in the container with the fish).
My Orandas get these every so often and although they are sometimes a bit unsightly they are a bit "par for the course" with Orandas and nothing to worry about especially if their behaviour is normal otherwise. They usually clear up on mine after a couple of days, then about a week or so later a couple more will appear and then disappear.
If you Google pics of Orandas, you'll often these pimples on their heads - especially on the darker varieties like Black Orandas.
As you mentioned, this must not be confused with what look like salt grains sprinkled on the fish, this is Ich (or whitesopt) and must be treated asap.
I'd see how he gets on and keep an eye on that lump over the coming days / weeks then, sometimes people panic and jump straight in with trying to medicate fish which is very admirable but sometimes it can pay to wait a few days and ride it out to see if the problem clears up, especially if everything else with him seems fine.
My Orandas get a few bumps here and there despite having no rocks or ornaments in the tank, just plants - those bumps can have a kind of cotton wool effect to them but they heal up after a few days or so, they chase each other round over now and then so I assume that's the cause, oh and wen growth too.
I think the key thing here is to ensure that he isn't exhibiting any other kind of strange behaviour that I mentioned in my earlier post and to ensure you keep your water quality in check, if it IS some sort of injury you'll need to make sure your water is tip-top otherwise that could lead to an infection.