Battling algae with a cartridge filter

I'm losing the battle with algae this spring. I've followed the forums advice and have had my chlorine levels high for days, but algae lives on. My water parameters are:

Total Hardness 100 ppm
TC 10 ppm (that's as high as my test kit goes)
FC 10 ppm (see above)
PH 7.8
Alkalinity 90ppm
CYA 50 ppm

I'm wondering if my filtration system is adequate......I have an 18000 gallon pool powered by a Pentair Supeflo 1.5 HP being filtered by a hayward Star Clear c1200. I seem to be cleaning my filter cartridge every third hour and my garden hose no longer cleans it, I had to purchase a pressure washer to get my filter's PSI back to where it ran last year.

Buy a better test kit that will allow you to test higher chlorine levels. We recommend the Taylor K-2006 which you can get through the testkit link in my signature. With a CYA of 50, your shock level would be 15 which you can't test with your kit. It could just be that you haven't made it to shock level and that is why you haven't gotten rid of the algae. In the meantime, you can force your kt to go higher with a dilution method. Not super accurate, and not meant to replace getting a good kit. Details here:
Testing Without a Good Kit

What are you using as your source of chlorine? Since your CYA is already at 50, I would suggest just using either bleach or liquid pool store chlorine. For a reference, in your pool, each gallon (4 quarts) of plain, unscented 6% household bleach will add approximately 3.3 ppm of chlorine. Try and keep your chlorine level at 15 while you are trying to clear the pool.

Also, run the pump 24/7 right now and clean the filter as needed. Hope this helps.

(Get the kit! It will make things easier for you!)

Welcome to the Pool Forum!

It's pretty normal for filters that are functioning well, to stop up with algae. PLUS, live algae plugs DE filters and, to a lesser degree, cartridge filters. Kill the algae, and you'll be able to filter better.