Doesn't sound like much of an algae problem. Although your test kits are probably ok...There are a couple posts I think @CalmSeasQuest made about the hanna phosphate checker that may be worthwhile to read.

If you aren't having too much of a problem with algae, I wouldn't worry too much about it.

GFO is your simplest and probably cheapest option (you can regenerate it). How often do you change it out?