Greetings, enthusiasts of Aegagropila linnaei! You have acquired at least one charming little ball of Japanese or Scottish lake moss, and you wish to keep it alive long enough to see it become a slightly larger charming ball of lake moss. This is the lowest-hassle and lowest-budget way to accomplish this goal. Embellishments outside the method described are pure shenanigans by people who want to sell you unnecessary products, or else they are overzealous caution by marimo owners repeating the "wisdom" of other marimo owners, with no effort to fact check or test other methods. This is, of course, very silly of them, since the simplest way to keep a plant alive is to emulate its living conditions in the wild.

Do

1. Acquire two transparent, colourless water containment vessels which are at least three times taller than they are broad, have a volume around 20 oz / 590 mL, and which do not completely seal at the top. The taller the column of water you are able to provide, the better. This facilitates convection of heat away from the marimo, which is native to cold lakes. The top of the container needs to be partially open to allow surface gas exchange, because marimo produce oxygen as waste. Having nowhere for oxygen to escape will slowly poison your moss.
2. Fill the containers to approximately 5/6 of their volume with regular tap water, leaving open air space at the top, also to facilitate surface gas exchange. Put the containers into your refrigerator, and leave them there overnight. The chlorine in tap water will offgas on its own. Place your marimo into one of the containers of dechlorinated water, and leave the other container in the fridge. (While chlorine on its own will not kill marimo directly, as organochlorines are naturally occurring in lakes with marimo, the concentration of chlorine in municipal tap water is still enough to stunt marimo growth somewhat, with continuous exposure.)
3. Place the marimo container where it will receive indirect sunlight for most of the day, and will not receive direct sunlight during the hottest hours of the day. It is fine for it to have direct sun at steep angles, such as during sunrise and sunset. Placing the container to one side of a window mullion, so that the mullion grants partial shade for most of the day, is a good choice on houses with only sunward-facing windows (south-facing if you live in the northern hemisphere; north-facing if you live in the southern hemisphere). You can place it directly in any window that faces away from the sun or on the east or west side of the house, as long as you live at a latitude greater than 36° north or south. Lower latitudes get enough sun that spending the entire day in any window of the house except one farthest from the sun, will kill the marimo with excess heat.
4. Every five days (seven days if you live at latitudes greater than 50°), get the second water container (which has been dechlorinating in your fridge) and transfer the marimo into it. Empty the other container, refill it with water, and put it into the fridge to dechlorinate. When you are making the transfer between containers, give the marimo a gentle squeeze to get old water and biological waste out of it, and roll it between your palms to preserve its spherical shape.
5. Do handle your marimo as much as you want. The moss feels really pleasant to touch, and a health marimo will emit a detectable "green" and pleasant fragrance when squeezed. You won't drown your marimo in air by having it out of the water for a few minutes. Algae survives out of water much longer than fish do.

Do not

• Don't fuss about filtering or boiling the water before you use it. If your municipal water system uses chloramine rather than chlorine, it will not offgas in the fridge, but it also will not harm marimo (chloramines are naturally occurring in lakes with marimo). Hard water also will not harm marimo. Filtering and boiling your water strips any useful trace minerals out of it, which will gradually malnourish your marimo over the course of a few years, resulting in dingy brown spots and slow growth.
• Don't supplement carbon dioxide in your marimo's water. Anything you add to carbonise the water will also make it too acidic for the moss (or in the case of baking soda, too alkaline).
• Don't add salt to marimo's water. Marimo are freshwater plants which derive no benefit from salination.
• Don't store marimo in opaque or coloured containers. The only colour filtration on sunlight they should be receiving is what water itself provides. Inhibiting part of the light spectrum, beyond this, is starving your moss to death.
• Don't keep it in a heated fish tank. Marimo is a cold water plant and will die in tanks with warm water fish.
• Don't leave it in the same water for longer than two weeks in any season, or leave it for longer than five days during hot summers. After this interval of time, surface gas exchange is no longer supplying enough carbon dioxide, and the marimo will "suffocate," and you can't artificially add carbon dioxide without making the water acidic or alkaline, as stated previously.
• Don't buy "plant food" of ANY form for your marimo - not even if it says on the label that it is for marimo. This is a waste of your money. Everything a marimo needs, it will get from sunlight, the trace minerals present in unfiltered tap water, and the carbon dioxide naturally present in a volume of water which has free surface air exchange. Additional chemicals, especially plant fertilisers for terrestrial plants, will poison your marimo. Biological waste from any cold water fish living in the same tank will have a negligible benefit or detriment to marimo growth, as long as the water is appropriately cold, but if the water gets too hot it will kill the fish and convert fish bio waste into forms which poison marimo (on top of marimo dying in too much heat, as well).
• Don't try to keep a marimo in a fishtank which has anti-algae chemicals in it. Marimo is algae and will die.

Never rely on marimo to filter or oxygenate a fish tank sufficiently to keep your fish from dying. Fish need active filtration and oxygenation. I don't care how "eco-friendly" you think you're being; relying on moss to keep your fish alive is animal cruelty, and sources which claim otherwise are unethical. Never keep betta fish and marimo in the same tank. Bettas are tropical fish and need a heated tank. Marimo is a cold water plant. One or both organisms will die in such conditions.

Troubleshooting

• If your marimo smells like sulfur, rotten eggs, farts, decaying plant matter, or anything strongly resembling these things, it means you allowed it to get too warm. It needs a cold water rinse, a "spa night" in the fridge, and a complete change of water.
• Don't panic if it has white, yellowed, or paler green patches. It got a little sunburnt, causing chloroplasts inside the algae's cells to "bleach" their chlorophyll or die outright, but this is not the same thing as the cells themselves dying. Give it a "spa night" in your refrigerator. Don't try to peel away the white spots; they may recover on their own. Algae is a colony organism, so cells with live chloroplasts can send nutrition to cells with dead chloroplasts, long enough for them to either recover or divide into new live cells.
• Don't panic if it has dark brown or black patches. Gently separate this fully dead plant matter from the marimo's mass, and gently squeeze all water out of it, then rinse it with cold tap water (yes, chlorinated). This inhibits further decay which might be caused by fungi or bacteria, or by getting too warm. Give it a "spa night" in your refrigerator.
• Don't use your freezer, or any containment in which water can become frozen, for marimo "spa nights." You also should not leave the marimo in refrigeration longer than a day and a night. Algae has a light-based circadian rhythm for its metabolism, just like any plant.
• Don't panic if the marimo starts to separate into more than one moss ball. Congratulations, you now have two marimo. Roll them gently into spheres. If this is contrary to your wishes, then use cotton or linen thread to wrap around the marimo and hold it together. Marimo is algae; the ball is not one solitary organism, but a colony of many individuals. Splitting is not dying or injury.

That's it: use two tall containers, swap every 5-7 days while the other dechlorinates in the fridge, give a little squeeze and roll, and keep your moss where it won't get direct sun or heat all day. Handle it as much as you want. That's literally all you have to do, as long as you haven't done any other things which can kill your moss. The steps described here may seem numerous at a glance, but marimo is by far the lowest-maintenance living "house pet" one can own, tied with air plants and succulents like Echeveria and Aloe, which have equally undemanding survival needs. If you need something less demanding still, get a pet rock, and if marimo is not quite interactive enough for you, try a Tamagotchi. Or attach some googly eyes to the outside of the marimo's container.

Iron Noder 2020, 18/30