Aranda's superb lob from outside the penalty area at the Ciutat de Valencia stadium put Zaragoza in the driving seat as they seek a likely meeting with Sevilla who are virtually assured a quarter-final berth after thrashing Real Mallorca 5-0 on Wednesday.
Betis remain on course to meet Atletico Madrid or Getafe in the last eight despite surrendering the lead at the Estadio de Gran Canaria.
Substitute Ruben Castro curled a first-time shot into the top corner in the 67th minute to fire the 2005 winners ahead before Macauley Chrisantus poked the ball over the line to give Las Palmas hope for the return leg in Seville next month.
Most of Spain's big guns played their ties on Wednesday when Lionel Messi extended his record goal tally for 2012 to 88 with a double for holders Barcelona in a 2-0 win at second division Cordoba.
Real Madrid fell two goals behind at La Liga rivals Celta Vigo but Cristiano Ronaldo's late strike made it 2-1, giving the 2011 winners a good chance of turning the tie around in the second leg at the Bernabeu.
Athletic Bilbao coach Marcelo Bielsa had a wretched night as his side were dumped out by third-tier Basque neighbours Eibar on away goals in a postponed last-32 second leg at the San Mames that ended 1-1 on the night and on aggregate.
Eibar will play Malaga in the last 16 with a quarter-final against Barca or Cordoba the prize for the winners.
Atletico have one foot in the last eight after Diego Costa scored twice in a 3-0 win over city rivals Getafe on Wednesday while Dani Parejo and substitute Roberto Soldado struck in Valencia's 2-0 win at Osasuna on Tuesday.