Rake back the weeds where they are growing strongly. This suggests no herbicides, and fertile soil.
Cost: Free.
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Safe-T-Sorb
40 lb bag does a 5' x 16" aquarium about 2" deep. Lots of coverage.
High CEC.
Looks good.
Find it at Tractor supply. See if the Grainger near you (if there is one) can get it. When I ordered it from Drillspot, it was delivered in a Grainger box.
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Sand has no CEC, so it is not 'the best' even if it was free.
It is denser than Safe-T-Sorb, so a 50 lb bag of pool filter sand will not cover as much as a 40 of STS, and they cost roughly the same (depends on where you buy it).
Lose-lose.