Tank stats:
37 US gallon, running for 7 months, weekly 30% w/c with light to heavy gravel vac
Penguin mini bio-filter
Penguin 170
Temp 72f
Ammon 0
Nitrites 0
Nitrates 5
Kh 10
Gh 12
pH 7.8
Few live, but mostly plastic plants, and painted gravel.
Inhabitants:
1 7" weather loach
1 bristlenose plec
3 marbled hatchetfish
8 flame tetras
5 cardinal tetras
2 penguins
2 head and tail lights
2 swordtails
2 cories, 1 julii and 1 bronze
The cories that have just died were 3 pandas, 1 julii and 1 peppered. The pandas were in and out of hospital tank with fungus; the last one to die seemed to be getting better back in large tank, then took a turn for worse & started hiding in strange places and acting listless. Put him back in hospital tank, & behavior turned stranger: he would be sitting there normally, then would flip around and spin in circles, then flop to floor and lie there on his back/side, panting heavily. He remained like this for several days, and I was hopeful. Sadly he died, and the same day, the other julii died in big tank. I had noticed a white fungus looking spot on his side a couple of days earlier and next day attempted to catch him to place him in hosp tank, but couldn't manage. After this futile attempt, I could no longer see the white spot.

Any thoughts?

