It could be a build up of calcium in your water ionizer. Electric water ionizers concentrate calcium in the outlet ‘alkaline’ water. When the pH of the water increases due to the process of electrolysis, then the calcium changes from soluble to in suspension. In this form it begins to attach itself in crystalline form to the output pipes – even though the unit has excellent self cleansing of the electrolysis chamber, it can’t clean the output pipes. You may need to do a vinegar cleanse on your unit. If the flow is reducing it is very important to do this cleanse as soon as possible otherwise you could damage the ionizing plates.
If you have a Delphi then you will need to send the unit to the repair service for a calcium cleanse.