Black Edges On Leaves

[catxx]

i've noticed on the plants in one of my tanks some of the leaves are getting black edges on them, and generally not looking so good.

it's not a PROPER planted tank, i'll add pics tonight when i get home.
it's at about 1wpg ish (7.5 US gallon tank). with a bunch of random plants, just seeing what survives at the moment, the ones i think i've identified are vallis, amazon sword, cabomba and java fern. but i may be wrong! all they live with is 2 red ramshorn snails, 3 otos and a betta.
i feed the plants with those little tablets you put in the gravel once a week after a water change.

i'll pics this evening, but any ideas?

 

[craynerd]

is it the plants going black or black on the plants, i.e algae?

 

[catxx]

i think its the plants going black, it doesnt look much like algae, but then algae comes in so many forms, and most of it gets eaten pretty sharpish by my snails and otos.