It is highly unlikely that food caused a pH drop (not impossible, but pretty unlikely). What I'd like you to do is post more information. Like the size of the tank, and the current inhabitants, and how long has the tank been cycled? Current ammonia, nitrite, nitrate readings? How frequent are your water changes, and how much do you do each time? What is the current KH of the tank and the KH of your tap water? Have you added anything new to the tank recently, anything including new fish, new plant, new decorations, etc. ?
The reason I ask these questions is because the end products of the cycle are acidic, which means that they will tend to lower the pH. But, the buffering capability, which is measured by your KH, will tend to keep the pH constant. If your KH is exhausted, the pH will slowly drop over time. The best way to refresh the KH is by doing frequent water changes. So, answer those queries and I think it will help diagnose what has/is happening in your tank.
