Raising Blue Gourami fry

[Nik_n]

I had my pair of blue gouramies spawn in my community tank back in January. I pulled the eggs out and decided to hatch them out in another tank. The tank is an 20 gallon plastic tub with a heater and hang on the back filter. I have been feeding the fry live baby brine for most of their lives, high protein flake, frozen cyclops and marine mix etc. 

The problem is that they seem to be tanking FOREVER to grow. They are putting on size incredibly slowly. I perform a 50% water change 3 times a week, and these guys are barely growing. I was told that fry can reach sellable size in 2-3 months but these guys are about 8 months old and nowhere near being ready to be sold.

Am I doing something wrong with the fry??

I had to leave the tank on the auto-feeder for more then a month and lost about 6 fish. I expected to sell them way before but I had no idea they are growing so slowly. Is this just a species thing or is it my upbringing??? 

I would like to get into breeding fish but if these isn't working and the fry take forever to grow, despite my best efforts I feel really discouraged.    

[Beardedbillygoat1975]

Can you post a picture of the fry and the parents? 
You're feeding plenty and good quality foods. Water change schedule is very aggressive but I have no issues with that. I wonder about flow - what kind of HOB do you have? Reason I ask is if the daddy are using all their energy to deal with current that’s less energy to grow. What’s the temp? Can you give us your basic parameters- pH, gh/kh, nitrates? 

[Nik_n]

I have no plants in the fry tank, thats why I do so many water changes. I'm planning to add moss and other floating plants to the tank tomorrow. I have an EHEIM liberty 75 HOB. It doesn't produce much flow but I can always make the flow slower. 

Parameters 

ph-7.6, gh-53ppm, kh-268ppm (18°) NO2-0, NO3-15mg/l ppm

Here are the pictures of the fry. Most of them have good colour but the size is what concerns me.  

[laritheloud]

It's hard to see size even from photos, but for what it's worth, I know gouramis grow incredibly slowly. Are there any juveniles of varying sizes? You can try splitting off the bigger ones to a different tank and seeing if the smallest ones catch up.

[Nik_n]

No, there aren't any different sized fry, other than a couple of mm. I figured as much, I could hardly find anyone who has done it before.