dead paradise fish

Question
Dec 14:  A new problem has developed which is causing me concern.  Of the 12 fish in my 20 gallon tank, I have 3 blue-green barbs.  One of them is swimming unusually:  bottom corner of the tank, head down 45 - 60 degrees, about 1/2 inch off the bottom, constantly wiggling his tail -- almost like a fast twitch.  This is unlike how he was a few days ago, and unlike his 2 friends.  I suspect something is wrong but don't know what.

When I fed them he came up for food and if chased he'll swim around the tank, but when left alone he goes back in the corner and does his fast head-down wiggle.  This strange behaviour is similar to the dead albino paradise fish in that both swam uncharacteristically (and a few weeks before that one my pleco stopped swimming and died).  

Yesterday I put in a new filter (my old one broke).  I did a 25% waterchange. Today the ammonia is still almost zero, KH=7 and N02 is a bit high 0.3-0.8 as the filter was out of action.  

Can you suggest what's going on and what to keep an eye out for?  I don't see any damage (although one of the other barbs might have poked him in the stomach when I wasn't watching).  I don't see anything on him, sticking out of him, or missing from him.

Thanks for any help.  I do have a picture showing all 3, and how he's different than the rest, but I haven't figured out how you post it.

Regards
Larry
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The text above is a follow-up to ...

-----Question-----
Today I discovered my albino paradise fish, that I've had for about 2 months, dead.  He was trapped under the water wheel however I think he must have died before that.

Yesterday I found him staying at the top of the tank in a corner, just bumping into the corner and back again, again and again and again.  It looked really strange.  Occassionally he'd gulp some air, but back to bumping into the corner.

When I fed my fish they all seeme to feed fine, including the albino.  However, he was swimming a bit head down and not as active as previously.

I checked the KH = 6.5-7 and ammmonia was about 0.  I have 11 other fish and all seem fine.  temp is about 78-80  (I have pleco's and a variety of barbs).

I'm new.  over the course of the past 4 months since setting up the tank I've lost a ruby barb, pleco and now the albino.  I don't see any unusual growths or actions from the other fish.  All the fish come from one very good store here in London Ontario.

the tank was started in august and we waited for quite a while to introduce fish.  we only did 2 at a time, and everything has generally been fine.  I do a 15-20% water change very 2 weeks.  20 gallon tank.  it's a petsmart tank and filter, so it is a combined one piece filter.

lots of aeration too.  he was about 2 inches, maybe slightly bigger.  he wasn't bugged by the other fish nor did I see him act aggressively to anyone else.

Hope this is enough information.  Any help is appreciated.
thanks
-----Answer-----
Hi Larry;

That's too bad. I'm so sorry you lost him. It isn't uncommon at all to lose fish in a fairly new tank though. Even when we are very careful as you have been it can still happen. It's really hard to say why he died. Fish can die for reasons that we never do discover. Organ failure, cancer, genetic deformity, and other diseases common to other animals and even people are possible. These problems can have the same symptoms so unless we have done a necropsy (autopsy of animals) we never find out which one. Just watch your other fish for a couple of weeks to be sure they don't develop the same symptoms. Once they seem okay, it should be safe to replace him.

At Your Service;
Chris Robbins

Answer
Hi Larry;

It's possible he is just the "odd man out" in the barb school. They should be in larger groups so their aggression is distributed amongst more fish. Sometimes two will gang up on the third one, especially if it's a female. I like to get them in groups of 5 or more to help avoid that. Provide more hiding places too, such as plants.

Look at the fish's belly and see if it's bloated at all. Look at the eyes for possible protrusion. If any of these symptoms exist it may have an internal infection of some kind. If you want to show me a photo, send it to;

AllExperts_Fish_Guru(a)yahoo.com

At Your Service;
Chris Robbins