Sorry if this is a little long, but being a noob, I didn't
want to accidently leave out any important details.

I have a 6-week old 1000 gallon pond w/ a short stream and
waterfall and about two dozen 3" fish. About 1/3 of the pond
is covered with water lettuce and hyacinths, plus I have one
lily, an umbrella palm, and a taro. The pond sits in a 50/50
mix of sun/shade (5-6 hours of sunlight a day). Equipment-wise
I'm using a Savio Livingpond biofall, skimfilter, and a Tsurumi
TMU-2 2700 GPH pump. Last night I returned from a week-long
vacation and noticed that most of the hyacinths had yellow
leaves (far more than when I left). The water is crystal clear
and the fish seem happy, but I also noticed nearly all of the
moss algae on the liner and stream rocks is gone. I checked
the water this morning and got the following readings: PH: 7.8,
Ammonia: 0, Nitrite: 0, Nitrate: between 0 and 5 ppm. I know
zero Ammonia and low Nitrites are good, but what should the
Nitrate level be? Before I left it was between 5 and 10 ppm.
Isn't a coating of algae a healthy sign or does it just mean
I have more than enough plants now for the number of fish?
Could the yellowing leaves on the lettuce just be due to age?

Thanks in advance!