planted tank - ph question

[midnight1]

i have a planted discus tank with driftwood- it is a 55 gallon with 2 - sponge filters, 12 - 2" discus, 5 cory cats, 6 or so bristlenose plecos

20 gallon water change 2 times a week
temperature approx 84 degrees
nitrates/nitrites/ammonia - 0 ppm
kh - 90 ppm
gh - my test kit isn't working so not sure
no CO2
lights on approx. 12 hours per day (2 - 48" strip bulbs for plants)

my ph is 7.4 out of the tap
my ph in tank in morning is 7.4 and goes up to 8.4 by afternoon

my fish are all eating well and don't show any signs of stress.

i guess my question / problem is how will my discus do with this ph fluctuation that happens every day...i know it is from my plants because my bare bottom tanks hold a constant ph of 7.4....if this is a problem other than getting rid of the plants and gravel how would be the best way to fix this problem...i can't afford a commercially made CO2 system but i could add a 2 liter bottle CO2 system ( i know these don't produce a lot of CO2 ). here is a crummy picture of my tank from my cell phone - hopefully good enough for you to kind of see what is in there plant wise.

72159

TIA

midnight1

[rbarn]

Save up for a co2 injection system with PH controller.
Discus will not be happy with PH bouncing up and down every time the lights go on and off.

[DerekFF]

That's a big swing from such a lightly planted tank. All the other parameters ok? I never get that much even with pressurized CO2

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