It sounds to me like there is a lot of nutrient buildup to create an algae bloom that is bad enough to give you problems. Try taking away some of the nutrients from the algae by using aquatic plants. Any nutrients they uptake will be that much less for the algae and may make the algae levels acceptable to you.
There are algaecidal products made to treat problems like yours that are affordable. Probably much less than you paid for the aluminum sulfate. The problem with them is when the algae is killed off the nutrients are released again and another bloom will occur unless you are willing to keep applying it.
