Hi all,

Over the last couple of week I have lost several fish, initialy I atributed it to old age but now getting rather concerned.
My tank is around 200L & has been totaly stable for over 1 year now. Had only plastic plants, a small piece of old bogwood & 37 fish in it as a peacful comunity tank. (Bala shark, common pleco, clown loach, white cloud mountain minou, 2 kissing gourami, 2 neon tetra, 5 sepae tetra, 5 halequin, 5 rummynose tetra, 5 otocynclus, 9 guppies).

3 weeks ago I added some bogwood (from LFS). I soaked it in tap water for over 1 month to reduce the impact & water colouration.
2 weeks ago added 5 white cloud mountain minou (mates for the one I had)
1 week ago noticed pale scales on the bala shark so added some Medifin
5 days ago lost one new wite cloud & one rummynose (2+ years old), took the white cloud to LFS & they replaced him.
3 days ago the old white cloud and a rummynose were dead in the morning & the sharks dorsal fin was damaged. I added more Medifin but the shark died that day.
2 days ago Water change (weelky)
Today 1 guppy & another white cloud died. Also the gourami are sitting on the bottom of the tank, they look ok except they never do that & are never so still.
When I did the water change I took particular care on the water testing before the change. Tested the Ph, Amonia, Nitrate, Nitrite & found nothing wrong at all.
Now I am concerned about the rest of the fish as I dont know what is going on. Could the Medifin doses cause it as they were close together? Could I have added a disease from the LFS twice ? Could this be due to the bogwood somehow? (As this water is slightly brown from the log despite my efforts.)

Any help on this would be much appreciated.

P.S. Sorry for the long post but wanted to give the full picture.