Looks like some sort of damage caused by fish or snails.
Looks like pleco damage to me too.
Do you feed them vegetables etc.? It looks like Plec damage, and they are little plecs.and otocinclus
It could be, if you have a lot of some ions (Ca, K) it can interfere with the uptake of other ions (Mg etc.).The leaves actually have curled up aswell, could it be too much ferts?
Blanched zucchini in the microwave for about 2 min is a favourite.
Just moved house and yet to get a microwave lol
Mine are quite partial to nettle leaves, pick the top few rinse then pour on water from a boiled kettle, job done.Well then...the old fashioned boiling pan for a couple of min Only suggesting zucchini because otos don't seem to go for any veg.
The other easy way is if you slice zucchini, wrap them individually and freeze them. When defrosted afterwards, they are soft and ready for otocinclus feast.