Heater :/

[ShaedM]

right i've just got a heater and i've never had a tropical or warm water tank, placed the heater in the tank near the filter and it switches on then back off after a few mins, its brought the temp up to about 24/25 i think..well thats what the thermometer is saying. i read somewhere that you should place the heater near a water flow so it doesn't just heat up the water around it etc..i've placed it near the nozzle that sprays out the water from the filter..my question is this, where about on the heater is the part that detects water temp? the top part or the part that heats up the water?

ive included pics

http/tinypic.com/view.php?pic=j950dt&s=7

http/tinypic.com/view.php?pic=2h57776&s=7


oh btw what water temp do Betta fish like to be in?