Use an electrolyte that suits the best to your HHO Gas generator design. The distance between the electrode plates does really matter.
A - For electrodes with little distance between the plates you should use distilled water with a very little of any of the catalysts mentioned below.
B - For electrodes with more space between the plates tap water won't work. So use distilled water with a little of the catalysts mentioned below.
C - For electrodes with a lot of space between the plates tap water won't work. Use distilled water with one of the catalysts mentioned below.
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1 - Tap Water - H2O (containing minerals, salts etc.)
Advantages:
Disadvantages:

2 - White Vinegar - acetic acid - H3C-COOH
Advantages:
Disadvantages:
Available at your local grocery store.
A good mix for medium distance electrodes: 100% vinegar with (only if necessary) some baking soda. (If do so be carefull, because the reaction will produce co2 and some other gasses!)

2 - Baking Soda ( Natriumbicarbonate ) NaHCO3
Advantages:
Disadvantages:
Pure Baking Soda might leave a brown tinted residu.

3 - Sodium Hydroxide also called Lye = NaOH
Advantages:
Disadvantages:
Pure sodium hydroxide is a white solid; available in pellets, flakes, granules and as a 50% saturated solution. It is deliquescent and readily absorbs carbon dioxide from the air, so it should be stored in an airtight container. It is very soluble in water with liberation of heat. Use with distilled water.
4 - KOH
Also called pottassium hydroxide.
Advantages:
Disadvantages:
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