Backyard Aquaponics
A place to discuss aquaponics

Thanks i have some folders but theres some i use all the time

Potassium deficiency tends to show in older leaves first, as yellowing around the leaf margins, and interveinal yellowing in areas immediately adjacent to the leaf margin (as described in the "Beans" section of the link FF provided). This is then followed by the leaf tip and leaf edges browning off… just like in the pictures you provided.

Potassium deficiency is common in young AP systems, especially in fruiting plants.

Magnesium deficiency tends to first show up as small yellow blotches, between the veins, across the entire leaf. These small yellow blotches between the veins then turn necrotic (brown and dead).

I know there are no pics of cucumbers, but have a good look at the pictures and descriptions under "Beans" in the link FF provided, IMO this most accurately depicts the classic symptoms of both Potassium and magnesium deficiencies. The link EB provided had some brilliant photo's, of cucumbers, with classic symptoms of both deficiencies.

Plus, don’t forget to check that your GB is draining properly, because the symptoms for plants with “Wet feet” are very similar to the Potassium deficiency symptoms. IMO you have one of these two problems.

Or they don't like the 2kg poolsalt in the water (1000L)

Yeh... Sodium toxicity will burn the leaf margins, but I would've thought 2ppt wouldn't effect cucumbers to that extent, although it's possible, especially if you've had a warm day.

Or i can add potassium-magnesium (30% K²O & 10% MgO)
But i don't know how much to add?

I used my brain and a perodic system to calc.
100g will add 25mg/l potassium and 6mg/l Magnesium in the hole 1000L

Do you think this will be enough ?