Oh I forgot to say that yes, you can use other citrus fruits. A friend with water crystals uses oranges. I tried oranges and noted a slight bitterness to my palate. I love the lemons and have not yet tired of their delicious flavour. And indeed you can use both!

Oh and I used lime once...

Sugar substitute for that lovely "light" drink? Unfortunately not. The kefir grains require the sugar (that's what they eat). Overall you could use just sugar water and the crystals would still do their magic. But the lemon and sultanas provide a very nice flavour and probably some added vitamins and minerals.

For minimal sugar you can let the sugar be eaten almost all up by the crystals, or even after removing the crystals just by the probiotic infused water. At just the right point it tastes like a dry champagne with most of the sugar gone. Go too far and it tastes like dry white wine. A bit further, I imagine...vinegar.

If this happens (it goes too far) apparently you can add a tablespoon of sugar and it is palatable again. --Thanks for that tip from H.F.!

It is a very healthy drink though and when brewed correctly, the alcohol percentage is very slight.