Be sure to read Stinky's sticky:
http://boards.cannabis.com/plant-pro...hy-how-do.html

I would recommend to get a digital Ph pen like this:
Amazon.com: Milwaukee Instruments pH Pen, PH600: Kitchen & Dining

For now, yes get the fish tank Ph test drops and find out what's your tap Ph. bring it to normal (6.5 ish) and flush and test the first runoff that comes out. If it's brown colored let it sit so it's a bit more clear and test it with your fish test kit drops. The color may be a bit skewed but it'll still give you the ball park. Yellow is way too acidic and blue is way too alkaline (base). Adjust it more in the opposite direction and flush like crazy 'til it's in range. Skip watering 'til they are almost dry, water normally (with about 10% runoff) and check again.

With the 24 hour lighting I gather you're still on veg cycle. You do realize the HPS is for 12/12 flowering, yes?

Keep us posted. Keep up the great work. :thumbsup: