you can put the marimo in club soda to get them to grow faster. but you can't put fish with them while doing this. this speeds up photosynthesis.you can do it for a week and then set in aged water for a few days before you put it back in with fish. i've tried this and i never noticed any difference. but i only did it for short periods of time. and have never gone over a week. the marimo did look healthier,but not really bigger. i would assume that longer periods would warrant more growth. they also like being kept cool. under 80 for sure.50-70 is ideal temp for them.
They grow so slow you might never see them grow larger than what you have. In Japanese waters where they originate it takes about 20 years for them to get to be the size between a nano and regular. 20 years! I have had my large ball for over a year and after it split it kind of let loose a mini nano (really really small piece). But now it's more of a pile than a ball. My nano is slowly splitting too but has not grown in the almost year I had it.
I think I like the nano moss balls better because they're smaller and a lot of them look really cute together!