Young RES's are more carnivorous than adults. At he ages he will eat more vegetation. Feeder guppies are a great food. Stick with a variety of foods. Even small RES's shouldn't be fed but about every other day. Daily feeding makes them less apt to eat what you provide. Continue to offer dark green vegetation or grow aquatic plants in his tank. Water should be about 80 degrees and then have a warmer basking spot. If he's not warm enough he won't eat and can't digest food. Turtles don't make friends with fish. I bet he will eventually eat the guppie. Maybe that guppie is faster and he hasn't been able to catch it. I strongly suggest you find a way to get your turtle into an outdoor setup in the Spring. Turtles kept strickly indoors are much more prone to health problems. Also, RES become quite large and need lots of space. Mature females will need access to dirt for egg laying. Even if not with a male. See this link for good care info.
GCTTS turtle care

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