So, I bought the Top Fin floating glass thermometer which I think is the best, the first one I bought for my 1.5 gallon tank works much better than the black strips I bought for my 5 gallon. So, I bought another one for my 5 gallon. The only difference was the suction cup to hold it was black instead of clear.
Well, as soon as King Harold our royal blue CT betta saw it in his tank, he literally flipped out! He would jump on it, attack it, race at it all while flaring the whole time. It was something alright. Needless to say, I switched it with the other one with a clear suction cup and King Harold was totally cool and happy, and fortunately my betta that had the clear cup thermometer had NO reaction to the new one with the black suction cup... go figure.
Either way, its a good lesson to always watch those bettas, they have such personalities! And, all different in little ways as different as their colors can be.
Now, if only I can get my 1.5G acrylic tank to be a consistent 78-80 degrees rather than the 5-10 degree swing it does during the day and night which ranges from 75 to 85. I might need to just put in a Hydor adjustable 25w that everyone thinks works great. My other 2.5 gallon and 5 gallon are finally consistent but I see how hard it is for the smaller tanks (i.e. why 2.5g and greater is just better). I have a baby betta in the 1.5G.