Don't put any wire into this thing if there are animals in it. Copper is poisonous to marine life.
But it is an essential micro-nutrient also. Probably OK to use stainless steel though or gold plated electrodes.
Since the pH value should not vary quickly
Seawater pH should not vary much second to second, unless exposed to extreme pollution - it has it's own buffering capacity. Freshwater (eg river) has little buffering capacity compared to seawater and pH can vary dramatically with pollutant loads. Aquatic plants can alter pH even on a day/night cycle.