Did you mean Borax as in 20 Mule Borax vs. boric acid?
So if I am understanding you right you have mustard algae and you are getting your FC up to around 20 and trying to hold and your CYA is 50. Pool math if I am reading it right, shock should be around 29. If this were my pool I would wait until evening and shock to a minimum of 30 and I would maybe assume my CYA could be higher, so say at 60 mustard algae shock is around 34, so I would probably take to 40. I would not maintain that level, just get it to that level and let it start dropping back down. I was having some issues with algae and slamming at suggested levels was not solving the problem for what ever reason, and it was not until I shocked way past slam levels did I solve my algae issue. Hope this helps.Yeah you read it right. Pool is crystal clear but I have battled this for years know and can spot the mustard algae in a heartbeat. I know it sounds crazy but I get the FC up over 20 and hold it there, but it drops VERY quickly. I'm adding 4 gallons every few hrs. Here's my DPD test method: 10ml of pool water, two tiny scoops of powder. Add the regeant and count the drops till it turns white, then divide by 2 (.5 ppm per drop) So if I put 20 drops into 10ml and 2 scoops of powder that's 10ppm.