Topic marine in general contains high po4 as compared to seachem which is around the same price.the only way to measure it precisely is to use a digital po4 meter
Not the same price up here. Can only get Seachem from Cape Town, with shipping adds 30-40% to the price.
Also the difference in PO4 is way to little to justify the change. If anything seachem has too much. 0.01 - 0.03 difference.
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Source: http://www.aquariumwatertesting.com/...lysis_0208.pdf