The tank looks larger than 16 gallons to me ,or that could just be a optical ilusion caused by the dark back ground. You should look into a new heater since a unstable temp can cause fish to die very easly. That is most likely the cause of your other angels death ,and it could lead to your current angels death aswell (as others stated already). Atleast you got a heater though because when I got my first fish tank (About 7 years ago) I did't even put in a heater so my fish were allways slugish ,and died pretty often. You may also want to consider a larger tank since angels do like space not to mention the larger the tank the more stable the water is so thats another plus. Craigslist is great for buying tanks I must say (make shure to look before you buy!) in my state (south carolina) a 55 gallon runs for around 100 dollars. They almost allways come with a stand ,and often include heaters,filters ect. although I normaly just go buy my own filter/heater while saving those for back-up.
Are you on a budget?If so is it a low budget?