My advice would just be to trim it frequently in a nano if you decide to put it in. Caulerpa can go sexual, and when it does, it basically disentegrates into the tank and releases a ton of nutrients, which could be a problem in a nano. Since it grows very quickly, this means you want to trim it regularly lest it put too much nutrients in the tank if it ever does this. The nutrients can cause a major
algae bloom unless you have an active skimmer. However, until it does this, it works to stabilize pH, keep down microalgaes, and absorb excess nutrients in the tank.
So... Just pinch off any parts that spread farther than you want it to. A caulerpa is actually a single celled organism, so if you want to trim it, pinch the portion you want to take off for one minute and then tear it off. I trim ours every other week in our tank. I think you're fine if you dont let it take over more than 10% of the tank. I expect that by trimming it every other week, you could keep it from taking over more than 1 gallon of water volume.
