( 100 litre tank )There is a mystery disease spreading through my tank, and i think it is something to do with my high
pH? I have just tested my water and my
pH is 8.5!

I do weekly 60% water changes and I have not lost any fish for months and months, until now

Only my platies seem to be getting affected, and I have lost 2 in the last 2 days, with another one looking pretty bad. They seem to get really skinny, and I mean REALLY skinny, it looks awful

they look like this for quite a while and then die.
Can someone please tell me how to lower my
pH?
It has always been quite high, (never THIS high...) but my fish seemed fine, so as they say, 'why fix something if it isn't broken' right?
Also any ideas on what this disease is? how to fix it? I have some internal parasite meds but I have cories so I cannot use salt... Do you think this disease and my extremely high
pH are linked or not?
Nitrite: 0.1
Nitrate: 10
Ammonia: 0
pH: 8.5