Whole Tank Getting Killled!

Yesterday I noticed the redside dace in my 20 high had a broken back and he was found dead this morning. I come home a few minutes ago and 2 of my 4 silverjaw minnows have white "bruise marks" on various spots on them. My Pugnose minnow has a small white spot, 2 of the 3 remaining redside dace appear to have a couple spots of white, and my last rainbow darter that's been acting strange for about a week is on it's deathbed with no signs of disease or injury. I tested water and pH is normal: 7.6, ammonia is 0, nitrite is 0, and 7.5ish. The only fish that show no signs of anything odd about them whatsoever are my 2 Greenside darters, 2 swamp darters, and 2 bluefin killifish. The Pugnose minnows, silverjaws, and redsides are all still acting normally as well and are swimming/behaving normally. I think since the silverjaws are benthic minnows maybe the rainbow darter is going "insane" in his last few hours and is attacking them but I really don't know what to do. I think I'm going to euthanize the rainbow. Any help is much appreciated!

Silverjaw Minnows:


Rainbow Darter:

Better picture of the silverjaws, they're large bumps in comparison to the body size so I'm thinking definitely not ich. Maybe got a wound infected?

The Rainbow Darter is dead.

The female Greenside who has been fine since I've had her is starting to be very seclusive and not coming out to eat anymore. I think I'm about to loose the entire population in this tank if something doesn't change and I can't figure out what's wrong.

Just euthanized the Pugnose minnow, it was floating on the top, breathing heavy, and had a broken back.

FishRetard said:

What do you have for aeration?


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Sponge Filter, been working great since I setup the tank. The rainbow darter and Pugnose minnow have been in this tank for a good amount of time now. Temp is 70-72 F if you were wondering.

xraycer said:

Looks kinda like fungus to me.

I remember seeing your tank shot from another post and was wondering with all the algae growth on the sponge filter if there's much flow moving through that thing. If there is poor flow, there would be inadequate O2 for the bb that would otherwise inhabit that sponge.

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Good idea, I'll clean the algae off the sponge.