Small brown patches starting to form on the walls

Question
Hi,
I have a 50 gallon tank with about 20 inches of fish in it and two 50 gal filters running.
I've had the tank for 6 years now and have never changed the water... I only add about 1/2 gallon per week because it evaporates. Most of the fish in the tank are over 2 years old. I never feed them by hand because I always overfed before so I have an automatic feeder now.
Now for the first time in 6 years I have some dark brown patches with a small darker dot in the middle of them growing on the walls of the tank.
Any idea what this brown stuff is? Could it be algae? I don't have live plants... I did switch to a different flake food that looks the same as the one I was using before.
Also.. I've fed them dried flakes since the beginning of this hobby. Should I try something else?
I have small fish... lengths range from 1 to 5 inches.
Any advice will be appreciated!  

Answer
Hi Andy: it is probably algae... everyone is having an algae issue right now...  Sounds to me like you are doing a good job... do you ever have your water tested for chemical levels?  If they are happy with the food you give them then continue to feed them... I would offer small treats though from time to time like live brine shrimp... hope this helps... dave