Help, I have a very, very sick gecko. Exactly how do I achieve an accurate temperature gradient of 75 to 90 degrees in a 20 gallon tank. Besides the under tank heater, I have a full hood with a socket at each end for bulbs and a fixture for a long bulb spanning the length which currently holds a Reptisun 5.0 UV bulb. Aside from the temp gradient I mentioned, my vet specified a Reptisun 2.0 where I have been using a 5.0; is this an important variation?