i know one guy who grew his natts to 13", but they were overfeed and just obese, and were in a very large tank, now the fact remains, Yes reds CAN be kept in 55 gallons tanks, and will live in them, Can they thrive in them< No, in fact they can't thrive in a 75, they can't thrive in a 125 ethier, you'll need a BIG ass' tank for them to live comforably, consider this, you have to live a a tank for the rest of your life, the tank is 4-5 times your length, and a little wider then your lenth, you are used to being able to go wherever you want, do you think you'll be happy in a tank now? and yes 2x your length would be great for the width of your enclouser, but it is no simulation to accually keeping the fish in the wild, so imo you should have a tank that is the biggest you can possibly go for the fish, and in my opinion, you should not, keep them in a 55.