Your plant appears to be Brazilian Pennywort (Hydrocotyle leucocephala). When older leaves show signs of interveinal chlorosis as your plant is showing, and leaves show signs of 'puckering', that would indicate a magnesium deficiency.
II. Symptoms do not appear first or most severely on youngest leaves: Effect general on whole plant or localized on older, lower leaves.
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 re-mobilized, 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, stems are brittle and branches have a tendency to curve upward. Stems are weak, subject to fungus infection, usually leaves drop prematurely.
A magnesium deficiency is caused by insufficient magnesium available to the plant / leaves as they grow. Magnesium is a mobile nutrient which means that a plant can move the nutrient to where it feels it is most needed, usually the growing tips and new leaves. When magnesium is 'stolen' from the older leaves the leaves show the interveinal chlorosis (normal green veins / lighter interveinal leaf material). That is because magnesium is a necessary nutrient for photosynthesis which gives the leaves their green color.
Why do you have a magnesium deficiency? It could be a lack of magnesium in the water column or substrate or it could be due to an excess of calcium or potassium. In your case, with a dGH of only 3.0 I would suspect there just isn't enough magnesium in your tank. Typically I would recommend just adding some magnesium sulfate (MgSO487H2O - aka Epsom Salt) to the tank to correct the problem but with the very low dGH you have I suggest adding Seachem Equilibrium to your dosing schedule. Do an initial dose of one (1) teaspoon per 10 gallons, thereafter add one (1) teaspoon per 10 gallons of new water added during water changes. This dosing should increase your hardness to a dGH between 4.0 - 5.0. It will also add needed magnesium along with calcium, maganese, potassium, and iron.
Once you start dosing the Equilibrium you should start seeing a change in about two weeks. The existing leaves will not change; any leaves showing the interveinal chlorosis or 'puckering' will stay the same or may actually decline in health.
Watch the new leaves as they are formed. They should not have any 'puckering' and as these new leaves mature they should not show the interveinal chlorosis. Other plants in your tank will likely improve as well, becoming greener and growing more quickly.
Questions, just ask! -Roy
Here are a few of other plants in aquarium- do you see them as suffering from deficiency as well?
Sorry, pics are not that great.
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That Nymphoides hydrophylla (aka 'sp. Taiwan') will look much better with more calcium and magnesium as will the Nymphaea (Lotus).