Ah no good it’s the best thing iv tried so far lasts forever doesn’t add to ppm don’t need much and never see the pesky gnats anymore couldn’t ask for much more 
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Put a thick layer of perlite on top. We all deal with gnats no matter what, but ive noticed ever sincd putting a heavy layer of perlite on top it really deters them.
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Clay balls also works 1” deep
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