You can use the 40g tank just fine you will just have to do a few mods. I am assuming that you have a screen lid. Get a piece of plexi that covers 2/3 of the screen. Then get a CHE and put it over the open third. This is how i currently have one of my BPs set up but in a smaller tank. You also can add a few strips more of heat tape or heat maps on the sides of the tank. Not as pretty but it will get the job done. You are definitely going to want to cover a majority of the screen though. You can switch to a tub but since you already have a majority of the setup with your 40g, you can spend a few dollars more to meet your needs. Just my opinion.![]()