Ok, so I'm posting this in the beginner section because it seems most appropriate here but I'm not a beginner (have reefs and other tanks currently and in the past). Anyway now comes why I'm stumped. I have two goldfish tanks set up, the one with the problems is my 75. Equipment and decorations are as follows: eheim canister filter, air pump, fine gravel, two fake rock structures, and one large piece of Mopani wood. This tank has been through a complete breakdown/sterilization process before starting fresh with goldfish. They are all common goldfish and were doing well for maybe a month. One day I looked in and about 5 of 10 of them were sitting at the bottom of the tank. The next day they were dead but not floating (if that matters). About a week or 2 later the same happened with 4 others. It all happened so sudden with mass deaths both times after them looking and eating fine. I'm calling it a tank "crash" but could it possibly be the Mopani leaching something? If you think its a crash too what would cause this? I do regular water changes at least once a week, more if tests show something wrong. I also have considered maybe something airborne like a cleaner or fragrance spray getting in the water but I have a small African clawed frog tank near this one and its doing just fine. I'm at a loss right now. If it was just one death at a time I could understand it but each time its multiple fish and all act the same and quick deaths. Any ideas? Thanks.