Long fin white cloud suddenly died

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I bought 3 long fin white clouds on saturday. They all appeared healthy and were eating as of this morning. When I came home, one was dead and stuck to the filter inlet tube.

Ammonia, nitrites and nitrates are at 0 and ph is 6.8. I did a 25% water change before I put them in the tank on saturday. Tank has a few amazon swords, a java fern and a water sprite.

One of the white clouds appears to be "protecting" the bottom back corner of the tank.

 

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I got a chance to take a good look at him under a magnifying glass and he has a gouge in him about 1/4 long by 1/8 inch high on the upper half of his body just below and forward of the dorsal fin. This is on the side that was stuck to the filter inlet.

The filter is a walmart brand 10 gallon filter, it is the same as a Marineland Penguin without a bio wheel.

Last night it appeared as if two of the wc's had paired off and the third was grazing around on the bottom.

I haven't lost a fish in the past year that I started keeping fish and am worried that a disease or parasite might wipe out the entire tank.