I’m guessing this is a magnesium issue. Crypts seem to need more magnesium than most. Remove all the damaged leaves as they’re no use to the plant and try upping your magnesium dose. This is easy enough to obtain as Epsom salts.
What fertiliser are you currently dosing and at what level?
Thanks to both. The ph hovers around 6.7/8 according to my hand held electronic stick (not sure how good it is) Also, the drop checker does turn green. No idea about hardness as I've no means of measuring it.
I dose CO2 supermarket 'All in one' fert. daily at the recommended rate of 5ml/40 ltrs., which they state each 5ml dose adds:
1.5 ppm Nitrate
0.4 ppm Phosphate
1.6 ppm Potassium
0.2 ppm Magnesium
<"Could well be">. The other option is iron (Fe). Iron isn't very soluble in hard water so often iron sources like <"iron gluconate or FeEDTA"> don't actually supply any plant available iron ions (Fe+++).