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I have had a ZZ plant for 1,5 years. It was already big when I got it. It grew a couple of leaves last spring. The old pot was changed in August 2020 and the new one is 12 liters with Lechuza PON substrate. Also, it was growing new leaves in December. Also, the ZZ is going to get a flower now.

I live in not a sunny place and now, in winter, there is little sunlight. At the end of December, I decided to use extra lights for the plant. I use 2 linear lights (85cm-12W-4000K-1000Lm and 105cm-16W-4000K-1400Lm), but the lights are bigger than the plant diameter and, I guess, only 60-75% of light flow reaches the plant. I used luxmeter app on the iPhone and it showed 300lx.

I watered the plant once every 4 weeks before using the light and once every 2 weeks now.

The plant dries out small old leaves.

Now I found some light spots on the leaves. I guess some conditions are not ok but don't know what exactly. Could you please help, I don't know what it is and what I should do with it now.

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    It looks like Lechuza Pon substrate is just rocks, with absolutely no organic matter to provide nutrients to the plant, leaving it dependent on fertilizer for its nutrients. So, have you been fertilizing? How much, and what ratios?
    – csk
    Jan 12, 2021 at 20:53
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    Yeah, lechuza is a mineral substrate with prolongated fertilizers: 350 mg/l N, 250 mg/l P2O5, 325 mg/l K2O, 0,08 mg/l B, 0,5 mg/l Cu, 0.7 mg/l Zn. I don't use any extra fertilizers.
    – Maxim Popov
    Jan 12, 2021 at 21:03

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I would put my bet on over watering. ZZ plants are a very low light tolerant plant getting them grow lights is not necessary for the winter or in general. The fact that you have doubled up the watering since the grow-lights have been installed, it is the possible root cause.

With ZZ plants it is enough to water when the soil gets completely dry. During winter with such a large pot it would take significant amount of time. Especially with ceramic a pot you have.

I have a smaller, terracotta (clay) pot for my ZZ which dries out much faster than a ceramic and with that I water my plant around every 30 day.

I would suggest to cut back the watering and wait until the soil gets completely dry before next watering. You could keep the grow lights they won't do any harm, just make sure not to burn the leaves, so keep distance between the plant and the lights.

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    @MaximPopov That is good news. What about the bulbs of the plant are they soft or mushy? That is were the plant stores large amount of water.
    – Bence Kaulics
    Jan 13, 2021 at 8:54
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    @MaximPopov Check out this answer of mine as well about root rot: gardening.stackexchange.com/a/56099/17229.
    – Bence Kaulics
    Jan 13, 2021 at 11:11
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    @MaximPopov Is the bulb wrinkly and shrunken? Maybe csk was right and the substrate is too rocky and the water drains right through, the plant cannot suck it up.
    – Bence Kaulics
    Jan 13, 2021 at 15:42
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    @MaximPopov I see, then reduce the watering and see what happens. The plant looks healthy in general roots, bulbs. This plant could tolerate underwatering very well, but over watering could harm it pretty soon.
    – Bence Kaulics
    Jan 13, 2021 at 15:54
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    And as I say it before, the roots were wet. My hand became wet when I was touching roots among them.
    – Maxim Popov
    Jan 13, 2021 at 15:54

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