Fire Belly Toad acting very weak, unable to jump properly-HELP!!?

I have two fire belly toads, one of which is acting so strange. He is always the most active and vicous when eating, now when he jumps, he flips over like he has an injured leg, but i cant see a fracture, and when eating, he barely notices the cricket. During the day, he barely moves and stays in one spot. When I bump him softley with a chopstick to check his health (hopping health) he hesitates to move. Very weak and sluggish- PLEASE HELP!, the water quality was O.K then I just changed it today, and his partner is VERY active and jumpy. I checked for mites too.