Woke up this morning and 10 fish are dead

Question
We have a community 100 gallon tank that we've had for 20 plus years.  A variety from 4 different types of spiny eels all around 12 inches long... to tetras, hatchets, guppies, do-jos, small catfish, pictus cats, small cats, featherfin cats, and many more.  I cycle the water 25% weekly and feed a variety of dried, frozen and live food.  Last week some tiger barbs seemed to have come up with ick, so I took them out and treated them for it.  It cleared up and I added them back to the tank.  All seemed well until this morning when I found about 10 fish dead (including a 14 inch peacock eel that we have had forever)  broke my heart.  All of the sudden there seems to be a black slime over EVERYTHING in the tank, that wasn't there before.  Its even covered the fish.  I drained the tank and put the remaining fish in another tank, and when I did, all the black slime came off of them and floated to the top.   When I pulled out my "sunken ship"  there was an overwhelming foul smell in it as well.  I'm baffled as to what happened to my fish, as I have never encountered such a thing in all my years having freshwater fish.  The Ph and nitrate levels all tested fine.  Its just this black slime that I have never seen before.  Please help me... I don't want to loose the rest of my fish, I've worked too hard to have them all die on me now. :(

Answer
Hi Marissa
You said you do 25% weekly water changes(good!), are you also vacuuming the gravel well when you do that?  Sounds like you're having a problem with what's called cyanobacteria, here's an excellent article on it:

http://freshaquarium.about.com/cs/maintenance1/p/algaebluegreen.htm

I've never personally dealt with this, but sounds pretty bad, and I sympathize if that is what you're dealing with!  Since you've done a cleaning, if that doesn't help, you may need to treat the tank with the antibiotic erythromycin which most pet stores will carry.  It'll either be as Maracyn by Mardel:

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=4838

EM by API:

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=16818

Or this stuff, and judging by your tank size is probably your best bet:

http://www.drsfostersmith.com/product/prod_display.cfm?pcatid=12101

The prices will be more at the pet store, but that website to those links is very reputable and I've bought stuff from them numerous times with no problems at all.

Good luck with your problem!

Christy