How often do you vacuum the sand bed right to the bottom? If it isn't being done regularly and if the sand is less than 4 inches deep, the sand could be trapping a lot of detritus which is leaching nitrates and phosphates into the system and fueling the algae. Reading back in the tank's history the nitrates and phosphate were slowly climbing, then there was the algae outbreak, and now there is enough algae to bring the water-borne nitrate and phosphate to low levels, but they are still being added to the system and growing more algae.
If the sand hasn't been cleaned to the bottom in a while I would start with just a small section at a time per week, say 10% or less, so that the system isn't destabilized too much. Then after a couple months once the whole bed has been initially cleaned, start cleaning the whole thing every two weeks or so.
I don't know your tank or your methods; this is just a guess on my part so take it with a grain of salt. Anyone else want to chime in on this?
Last edited by LifeIsGreat; 07-27-2020 at 12:18 AM.
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