The chemistry forum has some good articles that explain pH.
Before you get too far, how are you measuring pH? If you're using a test kit with a color chart, be aware - they're notoriously inaccurate. A much better option is the PinPoint pH monitor.
To expand on Icefire's recommendation, one of the more common causes of low pH is excess CO2. Easy enough to prove - simply take a cup of tank water, test it, move it outdoors, aerate it for at least an hour (airstone, pump or bubble box), then test again. If it increased, you've proven the cause to be excess C02. Especially common during winter when our homes are closed-up. Human and pet respiration depletes O2 and increases CO2
Simple aeration (using outside air) will likely help, but may not be practical. If you have a 'fuge (with macroalgae), an alternative is to run your 'fuge lights opposite of your display tank. This gives you the benefit of photosynthesis during times that your display tank is dark; it'll reduce "pH swing".
Using a buffer (alkalinity increase) will increase pH, but only in the short-term.
Calcification is more difficult at lower pH. You can mitigate it somewhat by maintaining your alkalinity at the top of the range (11 dKH or 4 meq/L)