"bullet Holes" In Red Ludwiga Leaves

[coolie]

I know I'm going to hear "iron deficiency" however, this is a tank I dose iron about once a week.
 
The red Ludwiga from the bottom up developed "bullet holes" in the leaves. This is as opposed to disintegrating leaves, or discoloured patches that turn into holes. They just appear as bullet holes, a few weeks after purchase and work their way up from the bottom of the plants eventually causing the leaves to drop off.
 
Any ideas at all?

 

[daizeUK]

I thought holes were usually a sign of potassium deficiency.  But carbon is usually a prime suspect too.

 

[coolie]

Potassium now added. There's plenty of CO2 in there.

 

[AeonMapa]

Are you sure they aren't bite holes?

 

[DrRob]

Carbon was first on my list, not iron. So flow would be my culprit if there's plenty of CO2. Did it get thick and bushy? I've found that occasionally with plants that do well, they grow so well that they block the flow around themselves and then die back again.

 

[coolie]

No - it was a new plant (I think it's one of those pre potted Tropica plants), it's shooting upwards but loosing it's lower leaves in the process so becoming tall and thin.

As I only add ferts once every couple of weeks, it's very possible that one or another fert had bottomed out.