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Nitrate and phosphate readings?
Without pic main suspect is iron deficiency but being as your not dosing anything you probably have more than just a shortage of iron. There over a dozen different macro and micro elements needed by plants. Those need to be dosed in relation to light levels and plants/species and uptake rate. Lower-med light tank with no CO2 needs a lean dosing of a complete fert like thrive at probably 1/3-1/2 strength probably every other day or twice a week.
Without pic main suspect is iron deficiency but being as your not dosing anything you probably have more than just a shortage of iron. There over a dozen different macro and micro elements needed by plants. Those need to be dosed in relation to light levels and plants/species and uptake rate. Lower-med light tank with no CO2 needs a lean dosing of a complete fert like thrive at probably 1/3-1/2 strength probably every other day or twice a week.