When mine were babies, I ended up feeding them floating goldfish food until they were big enough to eat the Reptomin turtle sticks. Even the baby turtle sticks were too big--they'd try, not be able to bite a piece off the stick, get discouraged, and give up. They grew just fine, and the water didn't get cloudy or smelly.
Keeping the temp up is key. They won't eat when they're cold. I'd read that for juveniles both the water temp and basking temp should be 82-86 F, so that's what I did. I use an aquarium heater for the water and both a basking lamp and ceramic heat emitter (for night) for out of water. Our house is cold enough that during the winter I kept both on during the day. Zoo Med makes a nice aquarium hood in various sizes called a Slider that has outlets for basking lamps, heat emitters, and a UVA/UVB light, which is essential for shell development and health.
I agree about the expense. We've spent easily over $1000 on these turtles. There's also a big time commitment. If I had it to do over again, I wouldn't--or at least not five!