It takes a while for the aluminum to build up in the soil to the point where it will damage the rhododendron roots. Rhododendrons have shallow roots, so there isn't anything you can do other than wash all the dirt off the roots and replant them where there is no aluminum. I seriously don't think you are at that point. Just stop using Aluminum Sulphate. Use sulfur or iron sulphate in the future. If you want the hydrangeas to be blue, sometimes aluminum sulphate is necessary since both aluminum and acidity are necessary. I would try to not use any aluminum sulphate on the hydrangeas also, but if they won't turn blue, then use some aluminum sulphate selectively on the hydrangea(s).

Here is a link that might be useful: How to grow rhododendrons and azaleas.

Thanks, I will stop immediately. I have heard that with my hard water it is unlikely that I will get blue blooms anyway on the hydrangea. I really should have done my research first, I know that now.