How to increase growth in rotala rotundafolia?

I have bad luck with any stem plants in my low light no CO2 tanks. My guess is that CO2 is extremely beneficial when keeping these plants. I'm like you though, and plan to continue not using CO2.

I have R. rotundifolia in my tank. It grows so fast I cut it in about half every week. I can tell you what made the difference. Two things: dosing fertz, I can show you pictures of the bottom half of the stems when I wasn't dosing, and the air roots (or the underwater equivalent) are everywhere. The top half are green and vibrant. The other thing is I do CO2. I went pressurized a few months back and that has made a HUGE difference also. If you aren't willing to do CO2, at least dose fertz, including something like CO2 booster.

More light will help UN. You want to use a 4000-6500K. Are you running a standard t12, or t8. If so they don't penetrate very well. How tall are the planted ones?

The reason the floating plants are growing better is because they have access to more co2.

I have this plant and it grows like a weed. I also have pressurized co2. I think more light isn't going to help a lot because what good is more light if your plants can't use it. You could try and dose Excell that will probably help a lot. Also dose fertz.

Hate to disagree, but I also have this plant in all my 10g shrimp tanks. No CO2 in any and it grows just fine.

Don't get me wrong CO2 will help, but this plant will grow without it if you have enough light.

I had the best luck with this Rotala with the following;It was a 29g.Light was a Coralife Aqualight Power Compact Fixture for Freshwater - Straight Pin - 65W - 10,000k,30" and a Coralife Freshwater Dual T-5 fixture -Colormax/6500k,30".For substrate i used red Flourite mixed with very fine gravel.I used two 2-liter bottles of diy co2,and fertilized with Brightwell Iron,Potassium,and FlorinAxis.Trimming and replanting this plant as soon as the tops reach the surface is key to this plants success.I'll include a pic of said tank.The rotala is growing on the left.Good luck...

do you dose ferts?

I've grown some nice R. rotundifolia with decent light (2.67 wpg T5HO) but the problem was I couldn't use CO2 and only was able to dose ferts (dry) minimally. Eventually it became a hair algae magnet. I've grown some with a little less lighting (2 wpg T12) but the plant itself didn't grow nearly as fast. Not every tank will balance out the same way but if you add more light, you may need to compensate somewhere else.