Im getting a bearded dragon soon and to start im going to put him/her in a 20 gallon long until it gets big enough for something bigger. My question is, can i buy a uvb thats big and will probably cover most of the 20 gallon tank and when i switch it be the perfect size for the bigger tank, or should i buy one that'll cover half the 20 gallon and one that'll cover half of the bigger tank. in other words, can i use the uvb fixture i plan on using for my adult tank on the 20 gallon until i move up to the big tank?