ph spikes

Please, can someone help?? I have a DWC system, 90 liter (total capacity), filled with 45 liter. Have two air stones and a fountain sprayer. The tap water here is 7.8 - I use PH Down or Distilled White vinegar to get the reservoir down to 5.2 I also use an A/B Nute formula from the only...

1st time grow; have a large container with 95 liters of water, two 5 gallon buckets with a plant each in them and I have a "CocoPod" grow system with 8 small pots. Both systems have "fish bubblers" and fountain sprayers on timers. the plants appear to be very healthy. Here is the problem. I...