Stem leaves, black stripes - what's the cause?

[Kezzab]

Hi all, would appreciate an opinion on this.

Pic below is heteranthera zosterfolia.

Ignoring the wee bit of algae, you can see the horizontal black lines across tne leaves, alomst like theyve cracked.

In a previous scape in this tank the same happened with rotala colourata.

What's the likely cause? My suspicion is Co2 but im not sure.

Anyone know better?

K

 

[Kezzab]

 

[jolt100]

My opinion would be mechanical damage.
My H.zosterfolia gets similar cracks if I trim it or catch it when water changing or in heavy flow areas . Often the black/purple bruising runs to the end of the leaf. If they are unsightly i just peel the leaf off trying not to damage the rest, or just pull it up and replant the tip.
Cheers
John

 

[Kezzab]

It does seem more prevalent in the area that catches the flow, and where I'd trimmed. Strong possibility. Cheers!