One of the keys to “filling the tank” is to make sure you account for the mass of what you will be submerging, so that you don’t overflow the vessel. As for weights, there are several vendors on eBay and Amazon.com offering weights that are covered with silicone which are quite good. I think I paid about $8-$10 for a set of two and purchased two sets.
Apples are floaters to begin with, and depending on the size of the apple, a one-pint mason jar filled with water should be sufficiently heavy when placed inside the bag to keep it submerged. You don’t mention what kind of “tank” you’re using, but you can get all sorts of things like pizza screens or cake racks at the restaurant supply stores that you can put other weighty objects on and use those to hold the food under water.
Many kinds of less dense pork items (cushion meat, ribs, tenderloin) will also float during sous vide cooking (and even the vacuum sealed bags may inflate, compounding the problem), and you have to make sure they remain in the water bath.