Marimo moss balls and zebra mussels

To be clear -- I am not at all implying that keeping one zebra mussel is advisable.

Linus, the big issue isn't actual an individual mussel. They're sexual, and if you really had one mussel in your moss ball, it would be a minimal problem (in theory, it could release gametes into your water, you do a water change, and those gametes meet someone else's bizarre choice of pet's gametes. Also, the DNA being shed into the water would screw with some detection programmes.) The problem is that there's virtually no way to ensure that you have one zebra mussel, nor to even ensure that you have zero. Zebra mussels have a fairly unique larval stage, known as a veliger. It's a very small, planktonic stage that can, and will, live in balls of algae. So, if you bought a moss ball, even if you quarantined it, it might be crawling with these things, and in the meanwhile, you're dumping larvae down the drain every water change (or dipping your net in and then using it in another tank). The veliger stage typically lasts for about 6-weeks, but can last for as long as 12-16 weeks, and I remember seeing one study that showed veligers can persist for a year! While a 6-week quarantine isn't unreasonable, when you're looking at 3-4 months and still a risk.... Of course, you also run the risk that during your quarantine period, the mussels have developed and are now deep inside of that moss ball where you can't see them, or in your gravel, etc. They are small -- I mean, a full grown zebra mussel will sit comfortably on my fingernail, and thanks to COVID stresses, I've bitten them down to nothing. :) (They /can/ get up to 2", but are typically less than 3/4"). They're a huge, huge risk. Freezing the ball for 48 hours will kill the larvae or any mussel, but I've been advising people to boil them for 15-20 minutes. Bleach will kill the larvae. As an aside, there's been some discussion about using copper (e.g., Cupramine) to kill these buggers, and a lot of people are all for it, as it could preevnt the loss of the tank. The veligers and adults may be able to withstand copper -- most will die, but it's not 100% effective. Don't do this.

How fast do the z mussels grow. Could someone keep individual moss balls in quarantine and see if any mussels show up?

Since you studied them, did you see any mention of ways to kill them? Like, will high ammonia levels kill them or will chlorine kill them, Or will high salt in the water kill them? I know plants can handle high ammonia levels so it would be fantastic if you could rid moss balls of zebra mussels and their larva with a high level of ammonia.

I haven't seen any, but the book The Death and Life of the Great Lakes, which is about the health of the Great Lakes, is a good read. It addresses, among other things, the unfortunate introductions of many invasive species, including zebra mussels, through trans-Atlantic shipping entering the Great Lakes through the lock and canal systems.

Centreville Aquarium has some for sale, I was there about an hour ago