Question: what do i do after lake valor in pokemon platinum?
Answer: You find almost all the lake water has evaporated. Many Magikarp are lying on the lake floor, but you can't fight them because they are too weak.
Head to the grotto near the center of the lake floor while battling with Galactic Grunts. ... 
Fly to Twinleaf Town, then go to Lake Verity to meet Prof.