I've noticed that the PH in my tank water tends to drop low. I've checked the Ph of my tap water and it's 7.4. Actually today, two weeks since the last time I checked, my Ph was at 6.0. Luckily I had some stuff to bring it up. But what exactly makes it drop? Is there a way to prevent it from dropping?
I've seen the effects of a Ph drop, and they're scary!

Someone told me the nitrifying bacteria in your tank that breaks up ammonia and nitrite to nitrate uses Ph to do its job. So overtime Ph drops. I don't know how to prevent Ph from dropping though. I've read people using crushed coral or baking soda.

How many goldfish are in your tank?How big is your tank,what type/size filter are you using?How often /how much water are you changing out?

I have 2 goldfish in a 20 gallon tank. I'm using an AquaClear filter 200/50. I usually do a 20% water change every 2 weeks.

You need to measure your kh (alkalinity) from your tap and in the tank, this is the buffering capacity of the water. Just as gollie said, the process of converting ammonia to nitrite to nitrate uses these buffers and if you are low the ph will drop.

It can be easily corrected by adding baking soda (that's all the buffers are). You have to be careful to adjust the levels slowly because it also affects pH, and drastic changes in pH can be very harmful to your fish.

Here is a link to Dataguru's calculator that will allow you to see how much to add for your levels of pH and kh.

kh and pH calculator

You need to be doing weekly water changes to freshen the water more regularly. If your kh from the tap is low and you are waiting two weeks to do a change (with a fully stocked tank) that will definitely cause your ph to crash.

Hope this helps.