Sick goldfish! please help me!!

Question
Thanks for your help!  I've been slowly learning that often fish "experts" aren't really experts - just salesman.  That is part of the reason for my too small tank issue - and unfortunately, I have no room to truly upgrade for about a year.  Next year when my lease is up, I'll be able to upgrade my tank - for now, it seems I have to replace it (it's acrylic, and this is the sencond tank - the first tank began cracking, and I saw this morning that this one is as well - NO MORE acrylic for me, so I'll upgrade to a ten gallon.  As far as the ammonia levels are concerned, about 3-4 months ago, when I first started a tank, the filter that came with it was defective - and I didn't know.  As a result, my water got pretty bad - ammonia was off the charts, tests came out black actually.  It took a week or two to figure out what was wrong and get it cleared up.  since then, the ammonia has basically stayed at 0ppm, .25 at most.  When I switched tanks (I want to say 2 months ago), it spiked at .50 for a few days, while it was cycling.  Since then it's been at 0 ppm.  My local fish guy seemed to feel that the .25ppm spike was probably due to dead fish in the tank - I think the 1 danio may have been dead in the tank for a full day before I came home.  As far as my filtrations goes, I have the eclipse tank with bio-wheel - personally, I'm not too happy with the filter, it feels like the pad clogs way to easily, but it does cycle the tank (I believe 4 times an hour).  I also have an ammonia filter as well.  I'll get my water tested again today, and make sure they test for everything.  I think I might actually take goldy to the store tonight - the one guy who I for sure know I can trust there (and yes, this is a mom and pop store) is working, and I'm sure he'll be able to tell if it's a disease or not just by looking at my fish.  I know it'll be stressful to him, but I really don't want to lose him, and these protruding bubbles have me a bit on the worried side.  Thanks for your help again!
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The text above is a follow-up to ...

-----Question-----
I have a six gallon tank with one goldfish.  I had other fish (3 danios, 1 irridescent shark).  While I was away over New Years, the shark died.  He had been hiding in a cave I have in the tank, and my roommate found him dead.  On Jan 2, upon my return, I found a danio dead.  The next morning, the other danios died.  Obviously, I've had my water checked several times since.  On the 2nd, my ammonia was at 0 pts per million.  On the 4th, it was at .25 pts per million.  today, it is at 0ppm again (I literally just tested it).  I can't test my pH at the moment, but I have never had a problem with it, and it was fine during the tests on the 2nd and the 4th (done at my local fish store).  I am on my 5th day of using metafix and pimafix as well.  My local fish guys suggested to use them since the cause of my fishes' deaths was unknown.  At that time, my goldfish was looking slightly sick - possibly with ich.  He was carrying his dorsal fin low, and had two thick white lines on his head, which looked like they were under the skin.  There may have been a few white spots on his dorsal as well, but it didn't look like ich as I've seen it before.  Now, as I was doing a water change (25%), I noticed he developed bubbles on his dorsal fin (not where I'd seen the spots).  It almost looks like blisters, or like the bubbles on bubble-eyed goldfish.  They're clear, just protruding from the side of his dorsal.  His head looks a little spotty, but still not quite like ich.  The white lines have disappeared.  This is a fancy goldfish, if that helps.  Do you know what it is?  Should I continue the medicine?  Should I take him to my fish guys tomorrow?  Please help me!
-----Answer-----
Hi.
6 Gallons is too small for goldfish or sharks, the sharks need well over 30 Gallons. Try to test your nitrite levels, because they are also very important. Do a 40 percent water change today, and do 30 percent changes every day for a few weeks. Poor water quality is probably the cause. Do you know what the ammonia has been in the past? If it has been quite high in the past it may be ammonia burns. If you are not sure he has ich it can do more harm than good medicating him for it.
I would stop medicating for ich until you are certain he has it. Also, Goldfish are coldwater fish, and Danios and Sharks are tropical. If the water is warmer than it should be for goldfish (50 to 68F) strange behaviour and physical changes will oven be noticed. Lower the temperature to what it should be. Do you have suitable filtration in the tank? If not, I highly recommend buying a filter to maintain the water parameters.
Good luck,
Kathryn

Answer
Hi again.
You're welcome.
I know, I often feel like a salesperson, but if I could say "don't worry all your fish will be fine", I would say that to everybody, but if I am to do my job I can't unfortunately.
It is true the fish can't survive in a small tank, but unfortunately a lot of unexperienced people, who think they know things, give wrong advice.
A 10 Gallon would be sufficient for ONE goldfish, but no more. They need a lot of space and are very messy, and need good filtration. Unless you really want to stick with goldfish, I would buy a 15 Gallon tank as soon as possible, and you can have 6 small Tetras, a few small Corydoras and a couple of Blue Rams. Even a small tropical tank can look very nice.
Good luck,
Kathryn

P.S. If money is an issue with buying a new bigger tank, Ebay is a good place to look for a bargain. You can get a decent tank for a quarter of the retail price, often less.