Parasites. Then you have to treat the whole tank. I've lost a lot of fish too.
I introduced ich to my aro's tank and discovered it only when it was too late. My aro was too far gone. To keep the bio-filter going, I dropped the filter pads into my fry tank...just like the total retard that I am. Nearly wiped out the tank of pbass fry that Matt gave me.
You would have thought I would learn, but I did it again in my 250g. Twice. And it's not cheap treating 250 gallons all in one shot...or for five days in a row.
Betcha I'll do it again too. I'll have a cool fish without a home and I'll dump it right in. I'm such a retard.