The point of turning off the lights at night is so that it is dark and the fish can sleep- they need it as much as we do. So, turning off the lights during the day would be pointless seeing as it'd be light out and they couldn't sleep. Believe me, investing in a heater will help immensely. It'll help keep your betta healthy and happy, which will reduce the likelyhood of him or her contracting an illness.
Gravel vaccums are needed to suck out the poop and mulm from the gravel that accumulates over the week between your water changes.
If you want, you can/ would probably want to remove the decorations and rinse them under hot water to get rid of the pathogens from the columanris. I personally would say remove at the very least the deocrations and gravel and rinse them in hot water, simply because columnaris is never fun, and I don't like risking it still hanging around in the tank.
I would do a large water change now, maybe 7 gallons to help clean the water too. After that, just go back to the regular 50% water changes once a week.
when you get a test kit, you are able to test for ammonia, nitrites, and nitrates. In the ideal tank, you want 0 ammonia, 0 nitrites and 5-10 nitrates right before you do your water change. appart from water dechlorinator, you need no chemicals to clean the water, you just do a water change. No need to alter water.
The Ph should stay constant where you're at, you don't need it to be a perfect 7.0. For example, up in NY, the water is super hard, but the bettas are acclimatized and used to living in hard water. Down where I live, it is really soft (6.7-6.8 to be exact), and the bettas are fine. Ammonia, nitrites and nitrates are bad, but can be removed by doing a water change and replacing with clean water.
If your tank is cycled, then a 100% isn't advised because it would take too long and kill the cycle. If it isn't you might want to simply tear the tank down and do a careful fish-in-cycle (do at least 50% water changes a week using a gravel vacc) while monitoring the fish and keeping the parameters down to an acceptable level until cycled. Once it's cycled, then you would probably want to get a betta. :)
Since your tank isn't cycled, I'd go ahead and clean everything with a vinegar soak that I described on the first page (also rub down the filter and toss the media). Before you break everything down, you might want to pick up some filter foam to put in the filter as you go to get a heater. When you're cycling, just be sure to leave the filter on constantly and not to toss the media.
If you end up getting the testing kit, it'd help you keep an eye on the water parameters that way you know for sure when it's cycled. :)
Okay, one last question. The tank has been set up for about a month and half, and has had a few water changes. (obviously not enough) Starting from when the tank was first set up, the fish were added slowly. Is there any chance at all that the tank is cycled?? Will I know when I get the test kit??
it is very possible actually that it is cycled then, so long as the filter was always running, the filter media has never been replaced, and you never did a 100% water change. If you get a test kit, you'd be able to tell. A cycled tank will have 0 ammonia, 0 nitrItes and (ideally) 5-10 nitrAtes. However, if you don't have enough water changes done, the nitrAtes will be higher than 10
That simply means the tank is dirty and that the nitrates are building up (while not as toxic as ammonia or nitrites, in high enough concentrations, it can hurt fish). To lower the nitrates, you'd want to do a water change :)