my aquarium

Question
I have had a fish tank for a number of years and never had any problem with diseases or anything else.  I change my tank twice a month and have kept the same fish in there for 3 years.  It is a 5 gallon tank and there is a minnow, a ruby red, and a goldfish.  They have lived together for 3 years and never any problems and now there is a fuzzy substance growing on my fish and the minnow and ruby red have died.  What am I doing wrong and what is that stuff.  I use a Finer-Shiner solution and it is supposed to take out all of the chlorine and help to improve the look of the scales by hardening them up and improve the color of goldfish and fish alike.  They get fed twice a day and no more than all of them can consume within a few minutes.
Please give me some tips on what is going on in my tank and what i can do for it.
P.S.  I have never thought of checking the pH levels.  I never have in the past and have had no problems.  How do I check the pH levels?  

Answer
Hi Shawna;

That's a shame. I'm so sorry you lost them. The tank is just too small for that many fish. Goldfish are deep bodied messy fish that need a minimum of 10 gallons each in order to thrive. When fish are crowded they become weak and get sick easily. They really are lucky to have survived as long as they did. If it was just the ruby and the minnow they would probably have been fine, but with the goldfish it is just too much for them all eventually.

The best thing you can do right now is change all the water to get rid of the white stuff. It is probably a bacteria or fungus that took over when the fish weren't feeling well. After that, change 25% of the water twice a week until you can get him a bigger tank. It will also help him heal from the disease. You might want to get a medicine to treat it. Look at the labels in the fish store to find a medication that treats his symptoms. Without seeing it myself I really can't tell you.

Hope he feels better soon....

At Your Service;
Chris Robbins