@angrygamer...because your landlord saw the tank and told you to tell the neighbor you are selling it, it sounds to me like he already assumes you have a pet that you are not telling him about and has chosen to pretend he doesn't know. I wouldn't worry about it. I think he would have pressed further to know what was going on if he was angry and planning on telling you to get rid of whatever you had in the tank. Unless he says something to you, I wouldn't say anything. Sounds dishonest I know...but he isn't stupid, he must have known you were using the tank for something. You obviously aren't the first person to hide a pet. I had several apartments when I was in college that my cats were living in with me and were not allowed. You do what you have to do to have your pets.