(where to start ?)...sick goldfish

Question
I purchased 2 goldfish and 1 snail in August of this year. I don't know the exact types, but one looks like an orange Oranda, the other is calico in color, and I think it is similar to the Shubunkin, and the snail was a white-ish snail with red eyes and a yellow shell.
((The lady at pet shop preffered sinking foods because of the air that fish can swallow  hurts them. So i fed them sinking granules and pinch flakes under water so no air was swallowed in feeding. (Every once in a while the calico would swim to surface,as it normally does, but then it appeared it would almost literally drink the air. I thought this was weird, but figured it was trying to pick up any leftover food that may have been on surface??)))

 The tank is 5 gallons with a filter that was used in a previous aquarium set-up. Everything was running smoothly until i cleaned the tank for the first time. About a week after, i noticed the snail didn't move as much as it used to. Then one day it just died because i found an empty yellow shell on the tank floor.   
Then a while later I noticed the calico was acting a bit "jerky" and sensitive to sudden movements. These past couple weeks his behavior has gotton much worse. His "jerkiness" started making him run into the walls and occasionally he would randomly take a nose-dive into the tank floor. The tank was dirty so i cleaned it out again. I thought he started to get better one day, but then it would start to continuously try to swim trough the surface or keep running into walls/ground, and it started to swim on its sides and even upside down repeatedly. The next day it was fine for a little while, then i noticed it just lying on the bottom of the tank, breathing heavey and would have these random ceisure-like episodes.
I moved him to a one gallon bucket (lack of another tank) (with no filter) because I didnt want it to get my orange fish sick too. I have also noticed the transparent scales look more like purple now more than white like they used to.
I started researching a little on internet and found remedies like adding sea salt and feeding thawed peas. I added a little less than a teaspoon of salt to isolated fish yesterday (have not seen any change) and i tried to drop pieces of the peas, but now i realized that it doesnt even eat any more, and it rarely even moves anymore.
I am wondering if i should return it to the 5 gal. tank because it is probably not getting enought oxygen. Is it okay to tranfer it from the salty water back to regular water.? I am gone most of the day so i am afraid if it dies while i am away, it will affect other orange fish (which seems to be acting healthy).

I read symptoms of comon goldfish disease and i think it has Swim Bladder Problems?
If there is any advice or knowledge you can share on any of this, please do. I am still even stumped on about what happened to my snail.
Also, will a single goldfish get lonely, will it affect health being a single fish.?

I know this is a lot. Any info will be appreciated.

   My Regards, Maddy.    PS: Should I get a NEW filter.

Answer
Hi Maddy
I don't think your goldfish has a disease.  I think the problem is the water quality.  It's recommended to have a minimum of 15 gallons for one goldfish, 30 for 2 etc.  And those are the minimums, actually larger then that would be better.  

The problem with your tank being too small, for one, goldfish can grow quite large.  The commons and comets have the potential to grow 18+ inches, and even the fancier varieties can grow 6-8 inches.  A 5 gallon tank just doesn't have the room for them.  Plus, goldfish put out a lot of ammonia naturally through their waste and respiration, so a larger tank is definitely a must for them.  I don't think adding a different filter to your tank would help.  The only thing that would, would be a larger tank(30+ gallons) or if you can't do that, return them to the pet store and get a smaller fish that would do well in a 5 gallon tank.

Also, are you using a water dechlorinator when you change the water?  The jerking movements sounds like some kind of poisoning-like from chlorine or chloramine in the tap water.  But the laying down, not eating, purplish coloration, that all sounds like ammonia poisoning.

For now, the one in the bucket needs to have some kind of aeration in the tank.  When a filter agitates the water surface, that is what oxygenates the water by allowing the CO2 to dissipate through the surface and the oxygen enters the water there.  He's going to suffocate in the bucket.  Probably best to move him back to the tank for now.  I would do 100% water changes in the tank daily until you can return them or get them a larger tank.

For the snail, they're pretty sensitive to water quality, so that would be my guess as to why it died.  I'm also putting a link on here, it explains what cycling a tank is, and that's kind of relevant to your situation.  It all stems around water quality and ammonia etc.  That's the first article on there, there's also one for ammonia poisoning and nitrite poisoning worth reading:

http://freshaquarium.about.com/od/startupcycle/Step_5_The_Cycle.htm

Hope that helps and let me know if you have more questions.

Christy