I've grown a bit concerned about my rainbows. This is my first time keeping them, and i absolutely love them, but i'm worried that their growth is stunted. I've had them for about a year now.

They have grown some since i first got them, but i'm pretty sure they are not as big as they should be and it seems they haven't grown at all for several months.

It is a 55 gallon planted tank. The pressurized Co2 keeps the ph a bit lower than what is ideal for rainbows, about 6.8. Ammonia and nitrate are zero. Nitrite about 20.

I have four red irian rainbows, four turqoise rainbows and four australian rainbows. Also in the tank are four siamese algae eaters.

The largest of my rainbows is about five inches. The rest are only about four.

Any ideas?

"Ammonia and nitrate are zero. Nitrite about 20."
You meant that nitrAte is about 20, right? 'Cause otherwise I think I found your problem. If nitrite really is 20 ppm then that's toxic to the fish and would stunt their growth.

"The largest of my rainbows is about five inches. The rest are only about four."
How big are they supposed to be? How long is their expected lifespan and how old are they now?

Growth is not a straight line.

Here is an example growth curve: http://www.uj.ac.za/EN/Faculties/scienc ... 0males.jpg

Do you see how they grow really fast as juveniles and then grow super slowly as adults? If you bought all of your fish as juveniles and they live eight years, I don't think it's abnormal at all for their growth to have slowed a year later. That maximum length you're reading is exactly that, a maximum. Yours won't get anywhere close until they are near the end of their life span. As juvenile or middle-aged fish they still have some growing to do. Check out that growth curve I linked to above. It's normal for the growth rate to dramatically decrease at this age. They will increase in size extremely slowly from now until they reach old age.