I started out with 15 molly fry that were born about a month ago. They lived together in a 5 gallon for about 3 weeks, until I moved them to a cycled 20 gallon a week ago. Since they have been in the 20 gallon, they have been dropping like flies. I have found at least 1 dead every day. Now there are only 8, and 1 looks as though it is about to die.
Before they die, they lay on the bottom of the tank, just breathing. The only symptom I have seen is on one of the dead ones, there was a white fleshy material coming from the center of its head. This made me think it could be hole in the head disease.
I used an old carbon cartridge from another tank to seed this 20 gallon. I floated it in the water. I have since taken it out. But since I was worried the carbon could be affecting the fry, I took the carbon out of the new filter and replaced it with an established sponge from another tank and a new bag of BioMax yesterday. I also did a 50% water change yesterday and added 2 tablespoons of aquarium salt. There was no carbon in the 5 gallon they were housed in before this tank, and none died.
I have no idea what is causing these fish to die! The only thing other than HOTH theory caused by the carbon filter, is that some of the plastic decorations or gravel were contaminated with something (even though I rinsed these before putting them in the tank). If you have any thoughts, I would appreciate it.
20 gallon tank, established for 1 month
2 tbsp. salt added yesterday
Currently houses 8 molly fry (1 dying) about 3/4 inch long
No live plants
79 degrees F
Filter: Penguin 100 Biowheel
Last water change: yesterday, 50%
Feeding: a small pinch of crushed flakes twice a day
Lights: 20 Watt fluorescent on for 8 hours
(API liquid test kit)