Youtube was cropping the video so I removed it... here's a frame though:
Notice the thick white strips that are filling in my blood parrot cichlids' lower fins.
I keep 5 blood parrot cichlids, two loaches, a 6 inch clown knife, and a 7 inch black ghost knife in an 85 gallon tank that is well-filtered and has consistently had very low readings for ammonia, nitrite, and nitrate. I do weekly 15-20% water changes and replace the filter material about once a week because it gets pretty gross at that point (I only replace half at a time to allow the bacteria to re-colonize the new material). The water does seem slightly darker than usual today, but I've been unable to test it yet. I haven't added any fish lately and have been feeding them the same amount as usual, which is a meager mixture of pellets, dried shrimp, dried blood worms, and frozen blood worms. Just about all of them take to all of the food, including both the knives... you should hear the ghost knife crunching on floating pellets. There is no sunlight and I keep the light on for about 12 hours during the day. I've had the tank for about three months now.
Any advice on what this might be and how to cure it would be wonderful.