If you're still running plants but no CO2, that may be your problem. Plants will use up available CO2 very quickly, then start using bicarbs from the water (KH). When this happens, pH swings will be broad and fast.
I had a similar setup where I was using a commercial CO2 yeast method, then stopped using it several months later. That's when my fish count went from 7 zebras, 3 cories, 1 otto, and 1 pleco to 2 zebras and 3 cories. Ammonia, nitrites, and nitrates were next to nil, and pH was about 7.6-7.8.
If the plants were using KH, there's no telling how far the pH dropped at night when they produce CO2.
(Source:
Freshwater Planted Aquarium Care and Maintenance: CO2 in the Planted Aquarium)