Why is the current draw dropping in water electrolysis

Aug 2018
We've done an experiment regarding water electrolysis. Basically, we just separated the oxygen and hydrogen gas in a beaker that contains a magnesium sulfate solution. We set the power supply voltage to 12.46V with an initial .09A and final .01A over 45 mins. I was wondering as to what causes the drop in current draw. I have my suspicion that the concentration of the solution is decreased since we're using copper wires, and there was a displacement reaction between copper and magnesium sulfate.