No they will need a varying diet. Depending on it's age it will/will not eat any vegitables. Try feeding shrimp, fish, earthworm, snail. Also it will need extra minerals/vitamins such as reptomin and also calcium supplement(this is done by using low fat cat/dog pellets and hydrating them in water with calcium added). When feeding anything that's frozen make sure it's properly thawed before feeding it to the turtle. When it gets older it will start to eat the floating plants so that's great to have.
When feeding the turtle you'll want to feed it in a separate tank/bowl (this should be filled so that the turtle can fully submerge). This is because they are messy eaters and you'll want it to poop there too. Else you'll need to change all water in the tank within 24h, this is because of hygiene and minimizing the risk of the turtle to get ill.
About your tank you should only have the deepest part about 2x the length of the turtle. Also make sure you have a part in the tank where the turtle can go up on land and bask. If you don't have any land there is a risk of the turtle drowning due to exhaustion (especially if you have filled 100% of the tank).