The water shouldn't hurt your plants as long as you have not been using the tank to rinse out chemical containers or similar. Most of the 'murk' will be an algae and organic by nature. A bit of rust won't hurt either.

Thanks - my wife was worried about damage to plants.

Probably best to play safe, Macready, and put the water onto the soil by the plants rather than over the foliage.

if you can empty it just treat the inside with thompsan or metal paint that you use on gates . better safe than sorry .