Isolated my male betta

Hello,

2 days ago started a 20gal tank with 3 female bettas, and also a male.
The tank is heavily planted. Now before you tell me I'm not supposed
to put a male in with females, keep in mind that everywhere you look
for advice about fish, everyone says something different.

Now, the fish were all going ok, the male was having fun chasing the
females, then he took a break. Then about 5 hours after getting them,
I found the red female had grabbed my male by the tail fin and was
spinning him around! So, I promptly put my male in isolation. He was
ok again after about 15 minutes, and remained so until today - he still
looks ok in colour, but now he has this cotton wool white stuff stuck
around his gills, which I think is a fungus. He has been swimming, but
mainly resting in his leaf (from one of the 20gal tanks plants) that is
floating in the tank.

I put some fungus medicine in the tank with him, but should I take the
leaf out? Is there anything else i should do, like put in some salt?

As for the females, well since the red one attacked the male, they were
being a bit nippy and fighting a little, so I went to the pet shop and
bought some more yesterday - they are great now!! So lovely to watch
too, very frisky and getting along great.

Thanks for help!