Betta

[neutron20]

I'm going to say this nicely "you need a bigger aquarium" for your betta fish. 5 gallons at bare minimum or it makes me a very angry individual thinking about a betta living in a 1.5 tank. That is the size of the milk jug that I buy for our family to drink out of.. and you have a fish living inside of it for it's LIFETIME. What if I told you that you get to live inside of your car for your life... it is exactly the equivelant amount of space for that fish to live in.

 

[Kehza]

I disagree with Neutron's post tbh.. Betta fish live in very tiny spaces in their rice paddies, keeping the water clean can be difficult in small bowls like this, hence why it is recommended to have larger ones to manage the water correctly. 
 
A 1.5 for a single Siamese fighter is fine, just don't overfeed it  

Perform weekly water changes on the tank, no more than 50% of the water at a time. Clean the filter media using only the siphoned tank water (tap water will kill the beneficial bacteria). 
 
When you do get a heater, make sure you get a thermometer of some kind, you can get ones that stick on the glass for a £1 these are very useful. A temperature range of 23 to 27 degrees should be fine.

Chances are when you get a heater, for small tanks like this they don't very often come with a temperature slider, if this is the case make sure you monitor the temperature of the tank, especially during mid winter and summer, quick temperature changes can be enough to kill a fish via shock. 
 
Oh and you don't want the filter media to be spotless either, a bit messy is a good thing because this is where the beneficial bacteria that convert ammonia and nitrites are kept.