First of all welcome to TPT!
Sorry to hear about your plant issues. You indicate that your dKH is 8.0 - 10.0.....do you happen to know your dGH?
I downloaded and enlarged a couple of your pictures. It appears that the leaves emerge fairly healthy looking, however as they mature they develop scalloped leaf margins, a hook near the leaf tips, and "cupping" (downward or upward) curling of the leaf margins. As the leaves mature they seem to lose color, develop holes of necrosis (dead tissue) and I am guessing possibly fall from the plant shortly thereafter. Is this correct?
Since it is the older leaves that seem to be most effected it is likely the issue one of the mobile nutrients. Mobile nutrients are nutrients that plants can move from one area of the plant (i.e. older leaves) to areas of new growth if there isn't enough of the nutrient available in the water column or substrate. Common mobile nutrients are: nitrogen, phosphorus, potassium, magnesium, chloride, and a couple of other micro-nutrients.
Based upon the symptoms I see, and without information on the dGH and calcium ppm, I would suspect the issue is caused by insufficient available magnesium (Mg). This can be caused either by insufficient magnesium in the water/substrate or another nutrient effecting the uptake of magnesium (typically too much calcium). Let's start by adding some additional magnesium to your dosing schedule. If it doesn't resolve the issue we will measure the dGH and ppm of calcium (Ca) and calculate the ppm of magnesium (Mg) in the water.
I want you to continue everything as you have been. Lighting, photoperiod, water changes, and nutrient dosing. The only difference is when you do your weekly water changes you are going to add some magnesium to your tank. Go to your local drug store and pick up some Epsom Salt (magnesium sulfate/MgSO4*7H2O). Get the cheapest stuff on the shelf with no perfumes or additive. Typically $1.50 per pound, get one pound to start with. When you do your next water change for an initial dose add 1/2 teaspoon of Epsom Salt per 50 liters of water in your tank. This will add 5 ppm of magnesium to your tank. Then when your do your 50% weekly water changes add 1/2 teaspoon of Epsom Salt per 50 liters of new water added to the tank. Do this for four (4) weeks.
During the next 4 weeks watch the new leaves as they emerge
; do not watch existing leaves they will not change and may continue to decline in health.
Do the new leaves look healthier? Greener? Possibly larger? Is growth a little faster? Now, as those new leaves mature so they stay flat, maintain a healthy color, and the leaf margins do not "cup" or 'scallop' as badly? If so you are on the right path.
Post to this thread as things progress so we can make adjustments as needed? Questions, just ask! -Roy
C. Interveinal chlorosis. Interveinal chlorosis first appears on oldest leaves.
1. Older leaves chlorotic, usually necrotic in late stages. Chlorosis along leaf margins extending between veins produces a "Christmas tree" pattern. Veins normal green. Leaf margins may curl downward or upward with puckering effect. Necrosis may suddenly occur between veins. Potassium or calcium excess can inhibit uptake of magnesium...magnesium deficiency
When the external magnesium supply is deficient, interveinal chlorosis of the older leaves is the first symptom because as the magnesium of the chlorophyll is remobilized, the mesophyll cells next to the vascular bundles retain chlorophyll for longer periods than do the parenchyma cells between them. Leaves lose green color at tips and between veins followed by chlorosis or development of brilliant colors, starting with lower leaves and proceeding upwards. The chlorosis/brilliant colors (unmasking of other leaf pigments due to the lack of chlorophyll) may start at the leaf margins or tips and progress inward interveinally producing a "Christmas" tree pattern. Leaves are abnormally thin, plants are brittle and stems have a tendency to curve upward. Stems are weak, subject to fungus infection, usually leaves drop prematurely.