Subject: How to safely turn clear water to black.

Hello

I am a magician performing a stage effect where with the swish of my hand ( and a secret dose of 
ferrous amonium sulphate and tanic acid )
The water turns black. 

Is there any other way to safely accomplish this effect without these chemicals???? 

These are the ones sold by magic dealers because a dose of oxalic acid turns it back to 'clear'
 I do not need the water to go back to clear. 
and I don't like using these chemicals. 

Any ideas?? 

Re: How to safely turn clear water to black.