Go to Walmart and see if they have an HTH 6-Way test kit (NOT 6-Way test strips). Also, buy a bunch of jugs of their 8.25% bleach, a jug of distilled water and a couple of boxes of Borax.

Your pump and filter are very much mismatched. The pump is too big for your pool and the sand filter is too small for that pump. The problem is that the pump forces algae through the filter without filtering it out. It is probably also blowing sand out every time you backwash so your sand may be low which also would affect the function of the filter.

Post numbers tomorrow and let us know if you were able to find the 6-Way kit. Also, tell us if the pool is green at all anymore or just bluish and cloudy. Run the pump 24/7 right now and backwash the filter as needed.