I have tried tumbling, not tumbling, high light, low light, nutrient-enriched, nutrient limited (nitrate and phosphate) and have failed miserably at growing Gracillaria parvispora. The end result is always the same - the algae turns from a rubbery red to a pulpy pale green and disintegrates.
Now for some totally unknown reason, I am not losing and actually may be growing a small batch - what didn't die from my most recent order from Indo-Pacific Sea Farms. I have it floating in a collander in my sump under 12 hours of 50/50 96W power compact. The sump water level keeps the algae about 8" from the light. There is very little water flow in/through the collanders. As an experiment, my Calcium Reactor drip is into one of the collanders (to see if higher CO2 content may affect it). No difference.
Anyway, there is no sign of decline on the remaining batch and there appear to be growth tips. The only thing that is different is that I have recently given up on nutrient limiting and focused more on nutrient balancing. I have to feed very heavily to keep my caulerpa from going sexual. Even with heavy feeding I have never been able to measure nitrates in the system (attributed to DSB in both the display tank and fuge and the caulerpy). I can measure phosphates. If I attempt no phosphate control, I get some cyano growth on my substrate. If I overcontrol (via a small fluidized bed reactor filled with ROWAphos), my caulerpa goes sexual. I am at a happy balance right now and the Caulerpa is growing and it seems so is the Gracillaria. I'm not sure how fast - or if it will stay this way or decline.
