You are only half dosing the tank so things will get worse before better, corys are not prone to whitespot, if you could remove the sensitive fish it would be so much easier.
Right if you think the fish are coping with the half med, increase it abit more gradually.
What's your temp at the moment.
Right if you think the fish are coping with the half med, increase it abit more gradually.
What's your temp at the moment.