Recommendations:

Next Jan 1st. take a sample of water and send it out for testing. I'd also have them test for heavy metal (copper).

This year I'd stock 20 pounds of Tilapia that are between 6"-9" in size and treat the pond within 5 days pre/post stocking the tilapia in the pond. I would treat the pond as follows: Mix 32 fl. oz. of Cutrine plus liquid and 2 25 gallons of water in a pump up sprayer. Walk around the edges of the pond and spray the solution on algae that is in water <24" deep. Then I'd get Cutrine Plus granules and apply it to the water that is >24" deep out to water depth that is 36" greater than your water clarity reading on a secchi disk. I'd apply the granules at the rate of 20 pounds per surface acre. You will have to calculate how many square feet of surface water you have that meets the criteria that I outlined above to calculate how much Cutrine Plus Granules to apply equally and evenly over that area.

That should fix the problem for this year.

Take another water sample for a test in August. Test during the middle of summer and again in the dead center of the middle of winter. P in the water is highest when the plant/algae growth is the least. The P will be reduced by the Tilapia (it takes P to grow fish flesh, and removing the fish in the Fall will remove P from the water).

If you want to treat the pond for P prior to adding the fish, use enough alum to make a slurry that uses 150 pounds of alum per acre foot of water. If there are no fish in the water, then you don't have to worry about a pH change.

Edit to ask how are you applying the aluminum sulphate?