Close to zero phospathe means what with what kit?
Regardless of the reading, intercepting phosphate with GFO before the algae takes it up is generally a good plan for dealing with hair algae.
I also agree that more herbivores of some sort is also a very good plan.
Here's a copy and paste of a recent post of mine on "figuring out" the cause of algae:
If you have excessive green algae, don't waste a lot of effort trying to determine "what" is causing it.
Many folks mistakenly think that a high level of nitrate or phosphate can "cause" algae to grow excessively. Or that high silicate causes diatoms.
These aren't really true in the sense that it is never one thing. All algae must get relatively fixed amounts of several different things to grow: a source of nitrogen (which might be ammonia or nitrate), a source of phosphorus (usually phosphate), iron, silicate (if diatoms are the concern), and some other trace elements (and, of course, light, space, etc.).
If ANY of these is sufficiently low, it can slow or stop the growth of algae, no matter high high the others get.
Also, the levels of these measured with a kit may be low simply BECAUSE the algae is growing fast and taking it up (just as growing macroalgae or using anATS can reduce nutrients).
So testing with a kit is never needed to tell you "what" the cause is, since the cause is always an adequate supply of all of them.
Always think of dealing with algae problems by getting the nutrients before the algae does, no matter what any kit reads for any of these nutrients.
A kit reading might tell you the magnitude of the problem, however, so it may be useful. For example, reducing phosphate from 0.5 ppm is a lot harder than reducing it from 0.1 ppm to levels adequate to slow algae growth. Bear in mind that algae may grow equally well at both phosphate values and getting it a lot lower needs to be the goal. You may see no benefit until it gets down to 0.03 ppm or less, and if there is a lot of algae taking it up, you may need to go below what most kits can accurately detect.
Both nitrate and phosphate can be driven low enough to prevent most algae, although phosphate may be easier since some ways of reducing nitrate may not reduce the natural level of ammonia, which could be enough to supply nitrogen to some types of algae.
There are many ways to reduce nutrients, including phosphate binders (e. g., GFO), growing macroalgae or ATS, denitrators, organic carbon dosing (pellets or soluble versions such as vinegar), skimming, etc.
Discussion of these is certainly beyond this thread, but remember that you can usually attack algae using any of these nutrients they need. The caveat to that statement is that some problems types of algae, such as bryopsis, might require the nutrient levels to be dropped so low to kill it that corals and other organisms we want may begin to suffer. Those pests may require different and possibly more aggressive solutions. Consequently, it can be important to try to identify the type of pest you have before deciding what to do about it.
