Yellowing can also be a sign of low Potassium and iron levels. Java fern and
Swords have leaves that die after a certain period of time too. The anubias
leaves last much longer. Some of what you are seeing could be natural die
back. If you are getting new growth from the middle of the sword plant that
isn't yellow, then it is most likely that you have old leaves coming to the
end of their life span.
This can be frustrating for newbies, they buy a nice plant and it looks
good in their tank for a month or two but it doesn't get enough light or
nutrients to grow new leaves to replace the old, and the plant slowly dies.
15 watts of light is on the low side for swords to grow well, so it may be
doomed to a slow death or will shrink in size until it is hardly noticeable.
The java fern and the anubias are low light plants that can almost be grown
in the dark.

"Iman Mostafavi" wrote in message
. ..
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