What's happening to my wisteria?

I have several Wisteria (Hygrophila difformis) in my 35-gal tall tank whose leaves are "lacy". That is, much of the green tissue between the veins is gone. This is happening mostly on the upper & ?older? leaves; understorey leaves are less affected. The same thing is happening to floating wisteria. It looks like they've been eaten, but I don't think I have any herbivores, though the oto's spend a good amt of time "cleaning" the leaves. I also have several amazon sword plants, but they don't have the same "problem". All the plants are growing well. Or... is this just normal for wisteria?

Here's the tank's inhabitants:
1 pearl gourami
2 harlequin tetras
3 oto's (the culprits?)
6 cardinal tetras
11-15 pencil fish (very secretive!)

Jaysn - No these have been established in this tank for several months, if not years. I purchsed the tank, plants & fish from a friend who moved overseas, and have since started several new plants from offshoots. The leaves that look like they're getting eaten are submerged.

New growth looks like the plants in the photo, but the leaves I'm concerned about are broader but have holes or thin patches. It's happening on about 50% of the leaves. I'm guessing the problem is caused by either 1) nutrient deficiency (solution: I'm starting to use micronutrients; N & P are fine), 2) low light (solution: I pruned about 50% of the plant biomass last weekend), or 3) scenescense (solution: I'll prune more often). It'll be a few-to-several weeks before I see results; I'll post a summary or plea for other ideas then.