Hello PPL! I had an issue about 3 weeks ago. I sold my 40 gl tank. When I emptied it I saw horrible water marks inside. When it was completely dried it was awful. I feared the buyer was not going to take it because of that.. When the tank was full it was not visible..He bought it anyway, he was in urgent need of a tank...Is there any product that can help reduce this glass staining? Do water conditioners work for this too? I now own a new 75 and don't want this to happen in case I sell to upgrade again. 
