Chronic low pH problems are usually caused by either insufficient aeration, or excess indoor CO2. To figure out which it is, take a cup of tank water and measure the pH. Then aerate the cup of water with an airstone for an hour and measure the pH again. If the pH rises, then you need more aeration in the tank - increasing your circulation or using a protein skimmer will usually correct this problem.
If the pH doesn't rise, then you most likely have a lot of indoor CO2. This is a common problem in Houston, especially if you are in a newer home. To confirm that is the problem, take that cup of water you just aerated and aerate it outside for an hour. If the pH rises, then you have excess indoor CO2.
To correct this problem, most people end up just topping off with kalk/pickling lime (as mentioned above). The other solution is to vent the excess CO2, either by leaving your windows open a lot of the time (not very practical around here) or adding a fresh air intake to your AC, which is what I did when we had our AC replaced this summer.