27 degrees C is a good water temp for a young RES. the basking temp should be 35 to 40, and preferably large enough that your turtle can move around to find the desired temp.
Real sunlight without the glass of a window is good. Your turtle needs the UVB to help its bones grow. Most of us have a UVB reptile light we use.
You don't really need to worry about the water volume being too large, like in a large aquarium. In the wild a turtle would be in a pond that is much larger/deeper than anything you can provide. When my son's turtle was that size, she was in a 20 gallon aquarium that was about 33cm deep. She was fine. Ideally, you want more space rather than less. It is easier to keep the temp consistent and keep the water clean if you have more water. The turtle will enjoy being able to swim. It doesn't need to be an aquarium. Many have used large Rubbermaid tubs.
As others have said, as long as you change the water frequently, you can use what you have for now, but your turtle will grow big fast. Eventually, you will want something larger.
Not eating for a few days, even when young is not a big concern, so long as the activity level of your turtle is still good. Tubifex have a high fat content and other reasons why they are not ideal food for a turtle.
The main section of this website has some of the best info on RES care on the web.
Feeding: http://www.redearslider.com/index_nutrition.html
Basking/lighting: http://www.redearslider.com/index_lighting_basking.html
And much more.
If your turtle is getting much less active, you may need to consider a vet visit. There are a series of questions about issues with a turtle that can help us give you advice. You have answered some, but the complete list is at
viewtopic.php?t=2142
You are a responsible pet owner. You have noticed changes and you are trying to get the info you need to help your pet. The habitat may not be ideal, but that is because you did not know what was needed. The good thing is you are trying to learn and improve things.
