A pH swing up to the 8s, should not have an effect on managuense (unless there is something else going on with the water in the tank). The lakes they come from in Nicaragua can have pH near 9, yet in the rainy season in smaller bodies of water they inhabit, can drop significantly. I used to add low pH rain water to my tanks to induce them to spawn (my normal pH was in the high 7s, low 8s)
Where i used to live (near Lake Michigan), pH and alkalinity would often skyrocket in spring during lake turnover when certain nutrients sequestered in the substrate over winter were disturbed and released.
You may wonder, how this relates to an aquarium.
Have you disturbed the substrate lately, after a long period of calm?
Vacuumed the substrate, after not vacuuming in a long time?
Have you added aragonite or a calcium based rock, something like Texas Holey?
These both could contribute to elevated pH.
A "good" pH is relative fr the fish you keep. Just because its 6.8 doesn't;t make it good, or bad.
If the water in your tap is 7.4, but drops down to the 6pH range, that may "not" be good.
It may be that the fish urine i(uric acid) n the tank is overwhelming the new waters buffering capacity to maintain 7.4, and you need more frequent water changes to maintain stability.