Why did the PH rise?

Four days ago I added 9 Spiral Vals to my 20 gallon freshwater tank. (those
are the only plants in the tank) I also added 10 ml of Leaf Zone plant food
as per the instructions. Before adding the plants and plant food the PH was
about 8.0. I just checked my PH again and now it's 8.4. What's going on? Is
the plant food causing the PH to raise? I have a bubble wand, a Penguin 125
HOB biowheel filter, and two pieces of drift wood in the tank. The substrate
is plain old aquarium gravel that I bought at Petsmart. I have added The
inhabitants are as follows:
5 Platys
3 Corys
2 Otos
18 Platy fry

Ammonia 0
NO2 0
NO3 2.5

Thanks

Karen


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