The Rahaja Lake is a temperate fishing location, accessible from Dua Ribu Lake's bulletin board. It's titled under "Oriental Lake Rahaja Tour". You need to be at expert class rank +10, and pay 500 fishing points to join the tour. You can only fish from the starting area, and you can't walk around the lake. Be careful about casting; if you cast too far, you'll hit the bridge.
The fish here are types of Japanese kois. They usually are medium-sized fish, and they're popular pet fish in Japanese man-made ponds and streams.