Do not refill your canteen from stagnant water.

I left the company because my career became stagnant.

Active, energetic, and dynamic are a few antonyms of stagnant.

The stagnant water smelled nasty.

methane is produced in the stagnant water in rice fields...

Unlike rushing water in a river, water in a pond is stationary and stagnant.

1 stagnant water or air is not moving and therefore smells unpleasant:

Few fish survive in the stagnant waters of the lake.

2 not developing, growing or changing:

The new investments are aimed at reviving the stagnant economy.

The housing market has been largely stagnant over the past few months.