Consider this page a starter guide on how to keep a little patio pond full of plants and one the most hardy, helpful (they eat mosquito larvae!), and easy-to-breed fish in the hobby. It won't contain everything, but I wanted to make enough of a care sheet for friends interested in the fry I am growing out, and maybe this will inspire you, too.
Medaka (Oryzias latipes) are a cold-tolerant species native to Japan. They come in a very wide variety of color and shape mutations, and many keepers are hobby breeders interested in genetics. Because they have a low-bioload (small poops!) they can be kept in smaller containers than other fish and, with enough plants, without a filter. They do not need heaters except in extreme cases, and can live without the aeration of a bubbler (unless it's really hot out). This means they don't require outlets, and are happiest with some outdoor sun (they do not do as well inside for most keepers).
In a world in which every hobby is ground down into affiliate links and an arms race of supplies and tech, keeping little fish in old buckets for cheap can feel really rewarding. All the links I have are just to show you the bottle or whatever of what I've bought that happened to work in case making decisions on marine weld or whatever would keep you from your own happy bucket of fish by the door.
While you don't need to buy a heater, filter, or a fancy tank, you will need to purchase some pretty non-negotiable supplies:
- a container: for adult medaka, this should be a minimum of three gallons. I inherited some unattractive plastic barrel planters for my adults, and they work fine. You can also find a more attractive ceramic planter (glazed all around the exterior, not just terra cotta) and plug the bottom hole with marine epoxy (it's not hard at all I promise!). Smaller containers are fine for growing out fry, just keep an eye on water quality.
- water conditioner: this keeps your fish from being poisoned by chlorine. Seachem Prime also neutralizes ammonia in the water for 24 hours, a feature I have used as a bandaid if something weird is going on with water quality. Add this to all tap water.
- substrate: (This one is technically optional, but with no substrate your planting options are limited.) I sift some potting soil into the bottom - about an inch - and remove floating bits and perlite (white pieces) after a day. I then coat with a layer of rinsed play or diamond blasting sand, let it sit for a day, and then do a big water change. If you're anxious about doing this incorrectly, watch Diana Walstad show you how hard it is to mess up. Or, if you love spending money, buy aquasoil (I don't!)
- a test kit or strips: the gold standard is the API master liquid test kit. It will last you for the rest of your life and is much, much more accurate than strips. However, these fish are very hardy in a wide variety of pH and have such a low bioload that test strips would be okay for them. You need to test for ammonia, nitrite, and nitrate. It won't be hard, I promise.
- plants:this is the fun part! I source mine locally (post in r/aquaswap or your local subreddit, or fb marketplace if you do that kind of thing) or via eBay. You can find lots of bundles for sale. Some that root into substrate and have worked well for me are ludwigias, jungle val, and temple, which grows out of the tank nicely as well. Hornwort grows quickly and helps keep things clean without needing to root in soil. Frogbit and duckweed are easy floating plants, but duckweed is hard to get rid of if it takes over.
- food: you can order medaka-specific food from the brank Hikari; they also sell a guppy food that people report their fish doing well on. I use Fluval Bug Bites since they were stocked locally. I crush them into a smaller panel with the base of a burned-out candle I keep by the pond.
- adults: if you are lucky, your local fish store might have some. If not, check r/aquaswap or a forum for your city. You can also purchase some online, buuuuttt..
- eggs: are the cheapest way to get medaka in the mail safely. I ordered mine from eBay and 100% of them hatched. If you order eggs or plan to breed them at all, order some methyl blue - more on that later. You can hatch them in tupperware without anything extra.
Once your container has been set up and given a week or two to settle, test it for nitrate, nitrite, and ammonia. Nitrite and ammonia need to be zero, a little nitrate is fine. If there is nothing living in your tank, it might not cycle (grow very important bacteria), so you may get zeroes all around.
In a new container, I would add no more than 6 fish at first to get that good bacteria growing. Test your water a few times a week - your nitrite might temporarily spike (change water) and then nitrate will eventually show up at low levels. Once a week, replace 25% of your water with fresh conditioned water at the same temperature (I let it sit outside by the pond for a few hours). Eventually, you can get to where rain tops it off for the most part. You do need occasional changes at least every few months to remove trace elements that came in with your tap water.
Feed your fish 1-2x a day. If you have algae, I would learn to live with it - your fish will eat it too! If you can't see your fish eat all the food you give them in a minute, it's too much.
Like... that's kind of it. Feed, top off water and test it. If all goes well your plants will go nuts and you can trim them back.
In winter, they will stop eating and don't need to be fed. The adults will survive with some ice (1" or less) on the surface, whereas I am bringing my juveniles in this year. I bring my plants inside and put them in a cute fishbowl under a regular incandescent light bulb until spring.
In late spring, you should decide if you want babies or not. Babies mean lots of water changes and tupperwares, but it's not too hard.
All you need to do is make a spawning mop (save your old green sponges and slice off the corner of a pool noodle - it will still float) and order some methyl blue to keep fungus from growing on your eggs.
Check for eggs carefully every day until late summer. They might be hard to spot at first, but you'll get an eye for them. I put them in a container of tap water (the chlorine helps the eggs out) with a drop of methyl blue, change 100% of the water daily, and remove unfertilized eggs until they hatch. Once they develop eyes (cool!) they are very likely to hatch and I start added a drop of water conditioner. Remove unfertilized eggs when you can tell them apart and separate them from clumping in the container in case a fungus grows.
As they start to hatch, have different container's ready for your different batches - big fry eat little fry. If you have a shrimpy little fry, move them with friends their size. Start feeding them the day after they hatch - all mine did well on Hikari first bites, which is basically a powder. There are way more complicated ways to feed fry, but I'm not there yet.
As they grow, add floating plants and test/change (tbh I just change!) water weekly with the water conditioner. When they don't fit into the mouths of the adults, you can put them in!
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