Possible algae question

Question
Hello, thanks for taking our question.  Since July 06, we've had a Calico Ryukin in a 5 gallon tank.  It is my 4 year old's fish, but I find myself being the caretaker most of the time. :-)  Nemo (of course) is a handsome Ryukin who has grown to about 3 inches overall (body and tail) and is very active and even playful!  

Here is our set-up:
Tank- a Hex5 with bilogical filtration system (I realize 5 gallons is a little small for Nemo, but it will have to do for now), we've added 3 small 'bioballs' to supplement the area available for bacterial growth.  The tank has a 15 watt incandescent lamp that is usually off until the evenings before the boys go to bed.  Typically the lamp isn't on for more than 4-5 hours each day.  The tank does sit near a window, but doesn't get any direct sun.  The exterior light is softened by blinds.

Water- we use RO water from our under-sink system.  Every 2-3 weeks I will change about 20% (1 gallon) of the water by vacuuming the gravel.  Usually, after the water has settled/filtered a bit, I'll change the carbon filter as well.  I also use a product called Cycle to fortify the biological system, about 3-5 mL added each week.  In the tank are 2 artificial plants in 1" of gravel. One tall and frond-like, and one short and bushy.  Also in the tank are some glass 'marbles'- brown, blue and black.  The gravel is a size small enough for Nemo to play with but not swallow.

Food- usually once a day I feed Nemo a couple blood worms and 1 or 2 peas that have been blanched and taken out of the skins.  Initially I fed flake food, but have since learned to limit that.

and finally... my question.
Lately I've noticed what I'm guessing is brownish-greenish algae growing on the fronds of the artificial plants.  Sometimes small areas of this also grow on the tank walls near the level of the gravel.
It doesn't seem to bother Nemo and is easily cleaned off the walls of the tanks and the plants.  I am just wondering if you have any thoughts about it?

Thanks so much,
Brian

Answer
Dear Brian,
Thank you for your thorough and complete letter!

Nemo has been cared for well. Although he could certainly use a bit larger aquarium as you know. A 20gal aquarium would be perfect. The one thing I think that needs to be adjusted is the water change schedule. It's strange enough that many types of algae can thrive in aquariums that get little light--but have high pollution problems. An aquarium can have clear water but the amount of dissolved wastes can be astounding and I would highly suspect this to be the reason of the algae growth. Likely it is thriving on the high levels of nutrients because all goldfish tend to give off more waste then even other fish of comparable size. Simply increasing the water change schedule is the best remedy. Try changing at least 20% of the water twice a week and wiping off as much algae as you can. Goldfish do best with very frequent water changes and make sure you use an aquarium gravel vacumm with every water change.

Just one other possible problem towards Nemo's well-being is the sole use of RO water. RO water tends to not be very healthy for fish if it is used entirely by itself. This is because RO water often lacks essential minerals and trace elements needed for all fish. It would be best to slowly start adding a little bit of regular tap water when doing water changes to slowly get Nemo accostumed to mineralized water which is best for him.

I really hope this helps and feel free to write with anymore concerns!

Best wishes and Happy holidays!
Karen~