UPDATE: For the past week or two I've continued to research and at the same time experiment with a spare tank i had laying around.
Tank: 2.5 gallon acrylic
Light: Natural sunlight from south-facing window. In February, direct sunlight from approximately 10am to 5pm.
Heat: Sunlight again. I don't have a thermometer in this tank 24/7, but test readings from time to time with one i have in another tank. I do have a digital air thermometer next to the tank which tracks daily hi/low air temps. Air temps range from 60F to 83F in my apartment (drafty window + direct sunlight causes 23 degree temp changes).
Plants: I was keeping anacharis for a while, but it's not doing well in my hard water (GH
both exceed 14). Just picked up some hornwort this weekend which is better suited to hard water, and it's growing like gangbusters. In fact, the hornwort seems to grow much faster in the natural sunlight than in my 29 gallon tank with 17w light.
Fish: none yet.
Add heater? it may just be a necessity during the winter months. I need to do take water temperature readings before sunrise and before sunset to get track min/max water temps. I don't think the 2.5 gallon tank will be able to hold as much heat overnight as a larger tank would.
Plant research - find more plants suitable to my water parameters.