I have an amazon sword, anubias, java fern and african fern in a 26 gallon
tank with a 15watt fluorescent light.

Everything other than the anubias is getting yellow leaves (old ones) and
holes are appearing in the leaves as well. I know I don't have that much
light but i get a lot of non direct light through the windows during the
day. I'm thinking fish food might not be enough fertilizer for the plants.

What's recommended to fertilize aquatic plants? Is there a liquid I can add
to the water, or something i can stick in the gravel near the roots?

Any suggestions would be appreciated.

-Iman