Unknown fish disease?

Question
Hi Chris, Over the past three days my fish have become very sick and i cannot work out why. I have a 20gallon tropical tank with a heater and fluval 2 filter. The tank has been set up about 3.5months. I had 5 platies (one had 4 fry now 2months old) 2 golden loaches, 3 khuli loaches, 2 danios, 1 bumblebee and more recently a male dwarf gourami.When cleaning out i do a 15-20% change by way of gravel hoover.About once a month i clean filter in aquarium water. Nitrite -0, Nitrate - 40, PH 6.5 (lower that what it usually is at 6.8. Ammonia - 0.

On Sunday i noticed that 1 danio had little white projections on his body which you could only see front on (not like whitespot), as he is fast moving its hard to see. He was the only one that looked different although one platy had a slight raggedy tail. I tried to work out what it was and thought he had slime or velvet so i put in the British king treatment for this. next morning he was much worse and i noticed some white patches on another platies tail. I thought i had diagnosed wrong and went to a pet shop. They thought it seemed like a fungal infection and gave me pimafix. i did a 15% water change at this point then put in the pimafix.

Within 30 minutes most of my fish were lying on the ground very lethargic. My golden loach was acting very strange moving quickly at the top of the tank and look as though he wanted to jump out. By the evening they all looked like they were gasping.When they do move the itch themselves on rocks. Now most of them showed signs of fins rotting and white patches (not spots).

The next day i went to another pet shop to explain what was happening. I bought oxygen tablets as i guessed the medication was removing oxygen from the water. I also bought melafix which could be used at the same time as pimafix. i was told the melafix should perk them up.

This morning the danio was dead along with a golden loach, a khuli loach and a platy (his moth was like he was gasping :( . The alive ones all look pretty lethargic and i noticed their gills look swollen. Worried about lack of oxygen i have now taken out 45% of the water and lowered the filter in the hope it will be more efficient in less water?? I really dont know what else to do i will be gutted to lose them. The symptoms seem to point at many diseases so i dont know what to do. And i fear that they will be overdosed.

Main Symptoms: rotting fins/tail, white patches on fins/tail. projections from body (however only seen in the danio), gasping, crashed on ground, itching, one shimmying, swollen gills.

Any help would be greatly appreciated. Thanks, Katie

Answer
Hi Katie;

It sounds like a classic case of a bacterial infection known as "Flexibacter columnaris". The spots on the danio, rapid gilling and the tattered fins are some of the most common signs. Here is a very good article about it with treatment options;

http://freshaquarium.about.com/cs/disease/p/columnaris.htm

Good luck...

At Your Service;
Chris Robbins