I use a mixture of water, alcohol, retarder, and flow improver. You can use just water but you may get a bit of tip dry.
2/3 distilled water
1/3 isopropyl alcohol 99%
1 ml retarder
10 ml flow improver
Mixing thoroughly is important so throw some paint mixing balls in. You can get them off of Amazon for $8 for 200. Also before you ever shake a new bottle up, open the cap and clean any dry paint off with a q-tip. You never know how long they have been on the shelf.