A heating pad..you know lol those things you put on you when you pull a muscle
.
You can sit your tank directly on the heating pad..it warms the water from the bottom up.
Directions:
(with betta NOT in tank!)
Put heating pad on towel. (so it doesn't scortch whatever it's sitting on ..the cover it came with works great.)
Turn heating pad to LOW setting.
Place tank (without Betta in it), full of water, on heating pad.
Pick a spot where only half the tank is on the heating pad.
Wait an hour and come back and check the temp.
If temp is too hot...move more of the tank off pad, if tank is too cold put more of tank on pad.
Wait an hour and come back and check temp.
If temp is still not as desired move tank again.
Wait an hour and come back and check tank temp. Tank should be at a desirable temp at this point. (if you are using a tank larger then 1 gallon..you may have to move to the MEDIUM setting if the temp is not coming up to the temp you desire)
Wait 2 hours and check tank again. If temp is still at the same temp you have found the correct spot (generally placing the tank on the heating elements (bumps in pad) is not a good idea..as these get very hot.
But
make sure to acclimatize your betta to the warmer water
before dumping him in the heated tank. Do this over an hours time.
To make this more simple....draw a line around the edge of the tank onto the bare heating pad. This way you know exactly where it should go once you remove the tank from the pad.
Put the same temp water the betta is currently in, in the tank and put the betta in the tank. Put the tank in the marked off section and let the new water and the betta warm up together.
Still make sure to check the betta every couple hours until teh temp is at a stable temp and stays there for a few hours. You don't want to boil your fish!
Voila
