In our story there is a 'healing lake' which is our Creo vis source.
We have to mantain its healing abilities, currently so we harvest only half of the vis. I want to make a magic item to substitute its natural ability somehow. People go to the lake and maybe carry the healing water away, too.
How can I do this? Every tricky idea would be helpful.
Tall order, since permanent healing requires ritual Creo magic and thus is not typically possible for any device. Perhaps a Cr or ReVi effect in the device to enhance the healing properties of the water?
The device could be some kind of orb which can be filled with the water. The Vim effect would maintain the natural magical property of the water for use away from the lake for a given time perhaps (say, Moon).
What does the lake in game terms? Heal all wound or give a recovery bonus? If it was the second option, it wound be easier to enchant the infirmary of your convent. Something that comes very handy anyway.
I don't know anything else about it. I suppose it is good to heal diseases, not wounds. Otherwise my older sodales would hold such water in the covenant.
Perhaps you should investigate why this lake has this powers: if it is an Divine blessing you shoudnt be able to copy it. If some ancient god lives in that lake and the effect is due to his presence, you should speak with him. If some old artifact changes the water, or if the lake is a regio boundary or or or... every reason wold change your methods!
Good metathinking Lucius! Stupid of me to have missed that process 
I think my SG didn't bothered with its background.
So this way touching the water would be the trigger for the effect.
And another effect would circulate the water in the item. Sounds good.
Hmm I don't see how you got all that from my brief comment, but whatever works I suppose.
I was actually suggesting an effect in the orb which would target the water itself (like a wizard's boost of sorts) solely to maintain its magical healing properties. Now, considering (as Lucius made so clear in his subsequent post) that I was operating under the assumption that what you meant by "healing" was the healing of wounds (although heling disease is not different in any significant sense) I hadn't thought of an activation mechanism but more a case of a continuously running effect (a magical properties "refrigerator", if you wil, to keep the lake water in the device magically active for Moon duration).
The more I think about it, you never did suggest that water taken away would lose it's healing quality, so perhaps my assumptions were foolhardy.
Seems to me your comment about circulating the water suggests you are actually looking for more than a handheld device, you want a whirlpool tub, don't you? lol.
Birbin's Magical Restorative Hotwater Hungarian Baths now open for public use lol.
My problem is firstly the trigger.
When the water is magical it affects anything it touches.
But if the target of the effect is the water... Is it possible to make healing water with hermetic magic?
I think if the orb affect only the water in it the water the magical part of the water increases very slowly. That's why circulating the water.
My first idea was making stairs to the lake and enchanting one step with healing bonuses. 
I wasn't speaking about "giving" the water any properties, merely using a Vim effect to keep the water from losing whatever powers it already possesses (or that portion of it that I thought you wanted to take away from the lake itself).
The trigger in that case could just be touching the orb to the person and saying a set of trigger words (or a word). Otherwise you could think of it like one of those holy water flinger things they use in the catholic church, just shake some of the water onto the diseased person to let the water do its thing.
A staircase leading to the lake is also a good idea, but then the afflicted need to climb it themselves which they might not wish to or be able to do depending on how sick they are.
In the latter case, I'm not really sure how you can get around the ritual nature of CrCo spells to enchant one (or more) into the stairs themselves. One possibility would be to simply put one or more enchantments aimed at giving people the strength/stamina to climb the stairs to get to the water. That much you could do with ReCo or even MuCo enchantments.
I should think Diameter duration would be sufficient to them up the stairs.
Ah I understand already.
I will investigate the powers of the lake. Maybe I can use it to other purposes, too.