Hi I'm new to fish keeping. Had a few black Cory that died when I first started a few months back. They died with white fluff on them. Then all fine for the last few month. Then all of a sudden my opaline gourami died (no fluff) and today my very young bristlenose pleco died with white stuff on him. (Pics attached) I did a pH test and it's 8. Ammonia and nitrate look all ok. Any thoughts? I just put in some melafix to ensure no other fish die. I have a 200l tank and to a 60-70% water change every 3-4 weeks. Any way to stop this going forward or what may be wrong?Any insight would be highly appreciated. Thanks