I got a goldfish from the fair and did a lot of research.  I got him a friend from the pet store and a 30 gallon tank with a really nice filter.  I have had them both for 4 months and in the last 2 weeks one of them started hiding in one of the decorations.  He would barely come out and eat.  I thought about removing the decoration from the tank to force him out, but decided against it.  I had to leave town for 4 days and left my bf to care for them.  When I got home he was still hiding. The next day (last night) he was out of hiding, he had almost completely turned white and could barely swim.  My other fish was bullying him and keeping him in the corner.  I decided the next day I would go to the pet store and get some medicine and get supplies to test the water.  When I woke up he was dead.  I am so sad.  I took him out.  I tested the water and everything came back good.  So I vacuumed and did a 25% water change, cleaned the filter and put in a new filter pad and put treatment in the water like usual.  Now what do I do?  I want to get my fish a new companion but I fisrt want to make sure he lives, even though he seems completely healthy.  If I do get him a friend how will I know the new fish won't have a sickness.  The way they keep common goldfish at the pet store, you never know if they will be sick when you buy them.  I thought about getting a comet because they keep them in better conditions at the pet store and there is less of a chance they could be infected with something.  What should I do?  SHould I keep the new fish seperate until I know it is healthy? Please help!  Thank you if you took the time to read my whole post!