first question I have is did you buy your tank new or used? because if your tank has been used for anything other than fish that might be your problem... another thing you are changing your water alot I would only add water to your tank when it is low and if you do reg. water change only about 10% water change each time dont do a 50% water change it stress your fish out you have to think that is there home you are changing they get use to the things that are in the water so when you change it alot they dont do as well. last thing I can think of since you are already using salt is I would use aquasafe as my water conditioner I think its the best kind hope this helps