What time are you measuring your PH? Is it at 7.8 in the morning before the lights turn on? If so, you are probably ok. Test the water for PH again when the lights first turn off and the PH will probably be higher. You can also slowly add a PH booster to increase PH, but if you are testing 7.8 before lights turn on, then I wouldn't use additives to raise PH. Just increase the amount of water for your pwc
. You should also list on here what your alkalinity reading or DKH
If your nitrates are increasing that fast, it probably does have to do with the coral food you are adding, which is increasing waste. It sounds like you may need to upgrade your protein skimmer and then do a couple big partial water changes to get you nitrates back down before they get out of hand. I think liquid reef just helps to keep up water parameters like alkalinity, calcium, magnesium etc, so I doubt that is increasing your nitrate.
So in a nutshell my recommendations are:
1. Test for alkalinity which could have an effect on your PH
2. Look into upgrading your skimmer
3. Increase partial water changes for awhile