Plants depend more on the PH and more importantly the level of dissolved CO2 in your water. If you're not fertilizing, adding C02 and have special lights, you're depending on the natural water chemistry of the tank for the plants to flourish. That's fine but every tank is different and you'll have to do some trial and error to figure out which plants will thrive. They will grow sort of slow without the stuff I mentioned earlier which again is fine as long as your

isn't a lawnmower like mine is.
Some things to try:
Anacharis - it can be anchored in your substrate or floating. It absorbs most of its nutrients from the water column, is fairly hardy and good for your turtle to eat.
Watter Lettuce and other floating pond plants - sometimes do well with a turtle, sometimes your Turtle thinks they are a floating buffet. The problem with them is they block light for lower plants so if you decide you want to do something rooted... these may not work for you.
Amazon Swords - Your turtle SHOULD leave these alone but they sure are fun to play on. They are fairly easy to grow, need to take root in your substrate and are hardy.
Java Fern - These are hit or miss... if you struggle with other plants give these a try. They seem to like higher alkaline in the water.