OK I'm sorry to say but there's some obvious problems with your set up and that's probably why they have got poorly.... so 2 parts to this thread... treatment and prevention
treatment, if they have white fluff all over it sounds like fungus, you can treat with pimafix, there's details on the bottle of how to work out the dose based on the litres in your tank, do a fairly large water change (40%) before you start treatment, if you have carbon in your filter remove it, then treat every day for a week, carry on treating even if they appear cured. Then do another big change when your done treating and you can put new carbon back in the filter if you like. However if you do have a carbon in your filter do a little research into why it's there and what it does, personally i don't think it's nescessary for every day filtration, but I won't bang on about that now, it's another issue.
OK Hopefully that should get rid of the fungus, although it may be too late for some of the fish.
prevention, well firstly is your tank coldwater or tropical..... cos you have both sorts of fish in there. You can't just stick at a temp somewhere in the middle and hope they'll all be happy, the comet goldfish need to be kept seperatly to everything else. So you've either got to set up another tank to keep these two sorts of fish, or re-home some of them. That's your first major descision to make.
Test results - if you have no results I assume you have no test kit... get one.... now, this is a vital part of any set up, how do you know if the fish you added are used to your water quality? or if there's soemthing not vastly wrong with your water?? Especially when you see signs of disease the first thing you should do is test your water. I recommend the API Master Test Kit. Ok so go get your kit, then post your results for ph, ammonia, nitrite and nitrate, then we may have more of a clue why your fish have got ill. Also as they are new arrivals rign the fish shop and ask what they're water stats are, if yours are different to theirs then that's probably why your fish got poorly.
Shovelnose catfish - they get big.... very big.... up to 2 foot if my memory serves me correctly, do your own research though and find out about these fish. But I strongly doubt if your tank is big enough for them so you should re-home them.
Silver Dollars, same again, they're not quite as big as shovelnoses' but they do still need a really as they prefer to be in groups of 6+ so you need room to house a large group of them. Big fish and lots of them means a big tank!
That's enough to start, it seems you need to do a bit more research before you buy fish as you've not thought through your choices for this tank very well. Please look up the fish you have an you'll soon see for yourself that they are really not compatible and suitable for your tank. Sorry to be so negative but I don't have much choice.
If I was you I'd get on with treating them asap, while your treating you have a week or so to research and work out what you want to do with your tank and fish, then when everyone is well again you can re-home them and sort things out. Do it sooner rather than later as more often that not unsuitable conditions can lead to diseases recurring.
good luck
