Hi sorry I dint realise how bad the photos quality was, it was uploaded from my phone.
Iv uploaded another from my PC, hope it’s a little better.
The plant in question is the amazon swords leaves. The veins seem very pronounced and the leaves almost look dry
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I didn't see any mention of water hardness, dGH, or mention of dosing a GH booster, Magnesium (Mg,) or calcium (Ca). Unfortunately your profile dose not state where you live so I cannot lookup the local water utility testing results. If you are using RO/DI water I didn't see it mentioned.
I downloaded your picture and blew it up; this is what I saw:
a) it seems to be affecting your older leaves
b) the veins are remaining green but you have interveinal chlorosis (yellowing); notice how the leaves have a 'net-like' look to the leaves with green veins (including the small veins) and yellowish/green leaf material between them?
If this were occurring on new leaf growth I would ssuspect an iron (Fe) deficiency but it seems to be happening on your older leaves so I suspect a magnesium deficiency. I do not see any of the characteristic 'hooking' downward of the leaf tips in the picture so your calcium level seems to be adequate, however watch for leaf tip hooking when you start doing Mg. I use MgSO4 (Epsom Salt) to increase my Mg levels, it is inexpensive and available on the shelves at your drugstore or possibly even your grocery. Adding 1/8 teaspoon per 8 gallons of water will add 2.08 ppm of Mg to your water and will increase the hardness by 0.48 dGH. I would start by adding 1/4 teaspoon per 8 gallons and see if your plants show improvement in color and growth. Then when you do water changes add 1/8 teaspoon MgSO4 per 8 gallons of water added. Keep us posted on how things progress.
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 branches have a tendency to curve upward.