I run a triple reactor, and the GFO is in the last one in the series, so the flow is not very strong. As a result I get no tumbling. However, I know my GFO works very well, as it quickly reduces my P04 to zero when I change it.
It usually lasts about a month (even much longer sometimes), and the clue to change it is when my P04 tests start to climb above zero.
I use the LR Hanna Phosphate checker, and it is much more accurate (measures in 2 decimal places) than other test kits that I have used prior. They would show zero long before the Hanna.
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