Hello. I need help with a disease killing fish in my tank My tank has 0 ammonia, 0 nitrite and 10 nitrates, temp 75, ph is 7.8 and kh is 4 and GH is 11 (just did the testing before posting), have a 55 gal with two Aquaclear 70 with purigen and matrix, airstone, UV Sterilizer and 5 goldfish, 4 tetras and 10 plants and gravel and has been running for 1 year. Last week one of my goldfish showed signs of sickness by sitting at the bottom and up top of the water, also 3 days before he died he was starting to be really skinny like something was sucking all the fluids/fats out of him. He didn't have any bent spine or skeletal deformities before he died and he ate until the night before he died. I saw white stringy poop in my aquarium before and along with the skinny disease and after reading online I thought it was parasites. I when to the LFS and got Seachem Metronidazole, Seachem Focus and Seachem Garlic Guard and Seachem Kanaplex (i was told to feed only for 3 days with Kanaplex) I mixed in with their pellet food as per instruction and gave it to the fish for 7 days and I have still 4 days left. But, there is another problem that started about few days ago which is I think Columnaris because of the hazy white film that I see at the edge of all of their scales and some white on fins and 2 of my goldfish lay on bottom sometimes and swim very slow with their fins around their bodies but they still eat. I had a molly in there as well and yesterday I noticed she wasn't eating, spitting out food and had the white hazy stuff a bit on her and I found her dead this morning laying against a plant and my husband thought she was sleeping... I called the LFS and they told me to keep giving them the food mix and also treat with Melafix/Pimafix for 7 days for Columnaris. I started treatment yesterday but 1 molly died so far and they all still seem to have it but they aren't doing worse either? Should I wait another day and then start with another medicine ? can you please give me advice on what I should do? I don't want to lose anyone fish this is affecting me so much I always tried to help them and do the right thing and this is so frustrating. Also, I would like (if possible) a medicine that doesn't kill good bacteria and plants.