Galvanic cell chemistry
The aluminum may not hydrolyze all the way to a neutral hydroxide.  It could form hydroxo complexes that remain in solution, especially if the solution is made acidic by the same hydrolysis.

Consider a half-reaction in which the Al is combined with only one or two hydroxide ions.

Some other tips: 

1)  Since you have an acidic solution don't put hydroxide ions on the left.  Put water and balance with hydrogen ions on the right.

2)  Sulfate ion is a weak base.  It will combine with hydrogen ions to make bisulfate.  So add sulfate on the left and incorporate the hydrogen ions noted above into bisulfate ions.