Checking the PH of the soil is a good idea, whatever the cause of this may be... Don't know about the quality of your tap water, so I can't say if it's good for your plants or not. Rain water is a better choice if you have a chance of using it.
You can try this: buy some mineral water (non-sparkling) and boil it for a min, let it cool down over night and use this for watering and see if it gets any better.
Slow growth can have a lot of causes incl. the variety you are growing itself, some don't grow very fast. What kind of lights do you use?