I'd say your tank is too small for a growing turtle. Why don't you look into an upgrade? The rule of thumb is 10 gallons of tank for each inch of turtle. Your turtle is already sized for a 35 gallon tank minimum, and she's only going to keep growing. There are lots of benefits to upgrading:
1. Your turtle will be happier. They love space.
2. Your water will stay clean longer because the turtle waste will be diluted in the larger volume of water.
3. With a cleaner habitat your turtle will stay healthier.
You also apparently need a stronger filter. I'm not familiar with a fluval, but if you go to a 55 gallon or larger tank you could use a canister filter like an XP3 which frees up space in the tank and will keep the water much cleaner.
ROCKS - if by "rocks" you mean gravel - yes yes yes it's dangerous for turtles to eat them!! Here are threads that show how awful it can be if they do.
http://www.redearslider.com/forum/viewt ... ght=gravel
http://www.redearslider.com/forum/viewt ... ght=gravel
On lighting and what to give your turtle to eat, please check out the info on these topics under General Information. It's very comprehensive. After reading, if you still have questions, please ask away. While you're at it, check out the other info too on basking, temperature, etc.
On the snappiness, I know 6 months seems like a long time but it really isn't in the life of a turtle (which live 20 to 40 years). He/she is still getting to know you. Always be gentle with him/her - don't add stress. I think if he/she is in a room with you for a large part of the day it helps to familiarize them. As they learn that you are the food source they will love you more and more. I pick Spot up to check out his shell every few days and by now, after 3 1/2 years, he doesn't try to bite me because he's gotten used to me. THAT SAID:
1. Some turtles have more aggressive personalities than others and will always tend to be more bitey.
2. You will always need to take care when picking up your turtle to keep your fingers out of biting range while holding him/her securely. They can always bite even after decades of familiarity.
3. When holding Spot I use a little wash towel because even though he's pretty good about not snapping, he does dig with those claws and I don't want to be startled and drop him.
On where to get an inexpensive tank, try Craigslist or ebay. Or do internet searches to see what's on sale.
On getting bored alone - no, turtles are perfectly happy living alone. In fact, a lot of the time if they are forced to share space with another turtle, they will fight and hurt each other. You should make the tank an interesting place for your turtle. Use live and plastic plants, river rocks (big ones), put minnows in occasionally (they're entertaining for the turtle as well as good snacks), vary the diet, put cuttlebones in. All these things will help keep the turtle from getting bored.
Good luck!