Can seahorses live in an aquarium with a red eared slider turtle?

No, and for a number of reasons.

First, seahorses are Saltwater Fish, and RES are freshwater animals. They could not live in the same environment.

Secondly, a turtle would eat a seahorse. Turtles eat smaller fish.

Third, seahorses are not available in many countries as they are a protected species.

Fourth, even if you were to have a seahorse, they are very difficult to feed and keep alive. They are recommended only for very experienced aquarium keepers.

Sorry!