Leaves turning yellow

Help!
I ws given some tiny cuttings, and now I have a jungle, but the tips of the leaves are turning yellow.
It's a planted RCS tank with about 3 watts per gallon.

I'm assuming that it's a deficiency of some sort, What can I use that won't hurt my RCS?
Thanks!

Thanks for the help.

Yes, just the RCS, even guppies were too big and eating the baby RCS.

Substrate is peat moss, enriched potting soil, and gravel over that. Two of the cuttings were planted in tiny pots (same substrate, just deeper)

I wish I could provide a pic, but the tank's in my office and my camera's at home...

Anyhow, the yellowing is just around the edges of the leaves. The plants have grown huge and are throwing roots down into the substrate from the offshoots. I think it is some form of ludwigia.

Oh, and I have used no ferts yet. The tank has been set up since December (I think)