fish diseases

I have had a 10G tank with 17 goldfish with no problems, no diseases, no
stress.

I recently got a 46G tank with the intention of getting clown fish but after
talking to the petstore I got discourage due to the high maintenance.
So, they sold me about 50 tropical fishes, from tigers to sharks, from
angels to colored ones.

Everything was fine until I notice white spots on the fish, spreading
rapidly to the whole tank.

The filter I got was rated for 60G.

I went to get ICK CLEAR, an effervecent alka seltzer type medication with no
luck.
After some research, got antibiotics (maraxin - ampicilin) which was very
expensive.
The petstore mentioned maybe I should get 2 filters.

I also got a Fancy Gold fish and put it in the 10G tank, after a few days
he got a hole in the head and die.

It's like these fish came with something.

To make it short, I swap the gold fish from the 10G to the 46G, and the
tropicals to the 10G to treat the disease.
I even swap the filters, using the 60G in th 10G.

Today, I only have 10 tropicals and added 3 gold fish with slight spot on
the tails. I think my mistake was to get too many tropical since they are
more sensitive than goldfish.

Also, what's the deal with the filters, they say something and do another.
I meant some says they filter 600gph however a 180gph canister is 2-3 times
moreexpensive.

To me, it's been a waste of time and money. How can I get rid of disease
on the tank? someone mentione sterilizer UV lamps, are they harm to humans?

- Will a canister filter be more effective? Why?

- Are these diseases harmful to humans? How about breathing the vapors? I
mention that because, I got sick (cold like)
since I start doing the maintenance. Water temp is higher than the GF
tank.

- Are sal****er fish less suceptible to diasease?


Good and helpful replies are welcome.

Thanks in advance.