Hello, this is my first post on this forum, so sorry if the topic is grim, but I need some sound answers. I started my first planted tank about a month ago. Because I had no experience I did not use any substrate only "pet shop gravel", luckily I have a friend who quickly corrected me on this error, and introduced me to natural tanks. I was really tempted to get pure natural river substrate, but because it was winter (you can't go dig for substrate in 1m of snow) I was forced to get "pet shop" substrate. Now in all the research I have done, I've never found any advice that says wash the substrate, so I didn't. I assumed that substrate especially this product I bought would not kill my fish, because it was designed to be used as aquarium substrate in a planted/fish tank. I added the substrate, then a layer of gravel on the top, and planted my plants, after this I waited for the water to settle and transferred the existing fish I had back to the tank (now with the substrate). I did this really slowly, so not to shock the fish. Unfortunately 1 of my black mollies displayed signs of stress, which has gotten progressively worse over the last 4 days. I did some research and found the source of the problem excess Nitrate in the water. I've been doing water changes everyday, and the water seems to be fine again, unfortunately the molly is not. All my other fish are fine, except this one molly which has been close to death for the last 2 days. it lies motionless on the top of the water for a while, then starts trying to swim, but it can't really swim in the condition that its in. Its almost like the poor fish is paralyzed. its a bit curled up, but its still very much alive. I decided to turn on the bubbles from the filter to give the water some extra oxygen, so that the poor Molly could stand a chance to recover. The question is - has anyone had a fish they thought would surely die, and didn't? When do you think is the right time to execute a mercy killing? I am not sure which is worse, I feel bad enough I basically killed the molly by messing with the substrate, now I can't do anything but watch it suffer. Do you think it could still recover? Thanks for your input.