The above is great advice to get you started and those are definitely the right first steps, but I want to mention something that happened to me and, surprisingly, I cannot recall seeing any other instances of this on the forum.
Beginngin of last season, I had a small algae problem, SLAMed, defeated it, all good.
Then, after a few days of no pool use and the cover on, dead algae would appear back on the floor. So, I thought I must still have a low level algae problem that I just didn't finish off. Removed ladders, checked the usual nooks and crannies, etc, passed OCLT, vacuumed it up. Then a few days would pass, same thing, dead algae on the floor (all the while chlorine consumption suggested nothing was living in my pool) After chasing algae ghosts for a few weeks, I finally recalled that I purchased a new cartridge media at the end of the prior season, saved $30 on Amazon vs. the Pool Store, great deal right? Checked for holes, proper seating, all of that stuff.
So, at my wits end, I ran to the pool store, purchased a new cartridge, dropped it in and boom, everything cleared right up. As another test, I opened this season with my discount filter (no algae needing to be cleared) and I just didn't have the sparkly water that I am used to as a dedicated TFPer. Switched back to my more expensive filter and sparkly TFP water restored.
Keep that in mind. Unfortunately, I do not see any obvious difference between the two cartridges and neither is there a description on Amazon about filtering ability (down to certain microns or whatever) that would allow me to distinguish, other than the media on one is white and on the other is more blue. No idea if that is at all meaningful or telling though.