Spring water is very variable & can contain quite a lot of impurities.
Distilled water is a perfect base to start with.
I would add 10% tapwater to distilled water & then use your regular fert.
If you don,t add the tapwater the Ph could drop like a stone when you add fert.
Your tapwater could be fine & if your plants are doing ok & distilled is not to expensive then go for a 50/50 mix.
Obviously getting your tapwater checked would be a good idea.