The fish does not look sick to me, they swim around, chasing each other.
Then some time some of the fish look tired stay near the water surface or
swim on the side way then they die.
I think may be I have problem with over feed. I feed then with the blood
worm this is a kind of worm is about 2 inches long, 0.5mm big they live in
water. Most of the fish breeder in Viet Nam use this to feed their fish and
my fish seem to enjoy this kind of worm.
If they die after changing water, then you must watch out for chlorine.
They might be dying due to suffocating. Worm of two inches in length is
too long. You can chop them to small pieces then feed them.