Because Otos are catfish, people believe they can be tossed into their tanks like other catfish and they don't need to do anything special. This is wrong. Without proper research into tankmates, food and water conditions you have a very high chance of killing your otos in the first week of ownership.
The first challenge your oto faces is the fish store. When they arrive at the store they are already stressed and usually starving. Then they get dumped into a tank without the proper food. If more than half the shipment survives its first 24hours you are doing well. If you have any left after the first week, these are the ones you wish to purchase for your own tank.
Never buy otos the day they arrive at the fish store because of this high mortality rate. Wait a minimum of 4 days after they arrive at the store, a week is better. In that amount of time, most of the otos who wouldn't have survived to be moved a second time will have died off.
Catfish books and Magazines
Choosing Healthy Otos
When picking out otos at the store to bring home, you have 6 things to look for.
Are they moving around the tank or hanging motionless for long periods of time? A healthy oto will be moving around often, it's mouth constantly moving over glass, plants, rocks, anything that is not another fish in the tank. If a net goes into the tank and the oto does not quickly move away, do not get that one. It may be a straggler from the initial die-offs.
Otos are very social fish, they love to be around other otos and if one swims off from the group to find a new food source, most of the group will follow until they are within inches of each other again. If the oto you are looking at is by itself, and seems to avoid the company of others, you do not want it as it may be sick or dying. They do best in schools of 3 or more, and the larger the school, the more fun you will have watching them dart around the tank at top speeds.
Otos almost always have their dorsal and caudal fins erect, even while eating and swimming. If you see one with either or both those fins clamped to its body, avoid it.
Healthy otos will have no red marks, or other surface injuries. Red gill covers, wounds, lesions, swelling, or anything that looks like the beginnings of sickness in other fish should be avoided. And if one has it, they may all have it so your best bet would be to skip that entire tank.
Healthy signs are round bellies without a caved-in look. They should have a slight bulge in the belly area (larger in females) to indicate happy and healthy fish. Since otos are constantly eating, this is another clue, if their bellies are caved-in looking and there is plenty of food in the tank for them, they may have parasites or worse.
And as always, even if you pick fish that look good in the store, quarantining them is still a good idea. They are particularly prone to ich and dropsy. They are also prone to handling damage because of their small size. Rough handling, and occasionally netting, can cause internal damage. The best way to catch these fish are with a plastic cup and use your other hand to herd them into it. This isn't easy as otos are Fast, but it is safer for the fish.
To help minimize the stress of bringing them home, bring a piece of sliced, skinned zucchini or cucumber with you and drop it in the bag with the fish before it gets closed. Then put the fish bag into a darker bag or container. Between the darkness and the food, the otos can relax more than without them.
If your local fish store doesn't carry otos, or is currently out, you can also buy them online. If you are very, very lucky, you might find them on eBay, but this is rare. You're best bet is to go to LiveAquaria.com they have hundreds of different fish and invertebrates, both freshwater and salt, and they ship nearly everywhere. I've bought from them before and have no complaints.
Male or Female?
Determining the gender of otos isn't as hard as you think. They only get to be 2inches long, with the females being slightly larger than the males. The easiest way to tell if you have a male or a female is body shape.
Is your oto slender (while still having a chubby belly) and more streamlined? It's probably a male. If on the other hand your oto is wider across the area behind the eye and in front of the dorsal fin, as well as being taller in this place, it is most likely a female. Another way to tell is from the underside instead of from above. If the belly is wider than the one of the oto next to it, it's most likely female.
Otos are found in South America living in the Amazon river. So it isn't hard to create a tank environment that they will like. If you want them to truly thrive and be happiest however, you should keep them between 75F and 82F. PH between 5 and 7.8 (though if you want them to breed, keep it below 6.8), with soft acid water. They also love currents as this helps keep the oxygen-rich water they prefer. They also seem to like hanging out directly under the filter where the current is strongest for part of the day, when first introduced to your tank.
Be sure to include lots of hiding places, as otos are easily startled. The females especially I have found will dart away at the slightest hint of movement towards them. They also love live plants. While you can have them in a tank without plants, they will be much happier with them. And of course if you wish to breed them, you lose almost all chance of this happening unless you have plants.
As well, if you house otos in non-planted tanks, you will need to feed them suppliments more often as the tank will not have the algae growth needed to keep them healthy.
Algae is of course the primary food. But if you don't have enough of it in your tank, you can also feed them algae wafers, nori, boiled romaine lettuce, peas (fresh or frozen), zucchini, cucumber, or green beans. Not all otos will like the same foods of course, my current set wont touch the algae wafers but go crazy for zucchini.
If they seem to ignore the food when you first put it in, do not worry. Sometimes it takes up to 2 days for it to register to them as being food. If after 48 hours your otos still have not so much as nibbled on what you offered, remove it. You don't want it to go bad or moldy in your tank after all. It could either be that they have enough algae to eat (check their bellies, are they still chubby and not caved-in looking?) or they do not like the particular item you are offering.
There are very few reported cases of otos breeding in the home aquarium. I don't believe it is because it is extremely difficult to do however, more like people either didn't do it on purpose, or just didn't bother documenting it.
Unlike most other fish, Otos need a densely planted tank, soft acidic water, mature algae growth, and clean clear water. They also do better in large groups. Lighting should be dim as they are nocturnal fish, and the should be twice as many males as females.
After they have gotten used to the tank and are swimming around and looking settled in, remove 20% of the water. Do not replace it for about a week. At the end of the week, add the water you removed back into the tank, but be sure it's a few degrees cooler than the current tank temperature. This should induce spawning behavior.
Your first signs should be a game of 'tag' between two males and a female. The males will chase the female around the tank, until eventually you see the pairing form a T.
Eggs are small and transparent jelly-like masses which will cling to whatever surface they were laid on. Most will be laid on plants, leaves and stems. Sometimes they are laid on the glass of the tank walls, but those ones don't always hatch, either they get eaten by other fish in the tank, or sometimes they grow mold from being exposed to the brighter light.
Hatching takes around 2 days, after which you can see the fry hanging on the glass and darting around much the way the adults do. Adult otos will not harm the fry, so there is no need to remove them once the eggs are laid. As long as you have a mature tank with plenty of algae, you do not have to worry about what the fry will eat, they'll have plenty of food for a while.
If you do nothing from the point the fry hatch, you will have a mortality rate of around 50%. To keep more of them alive, do 10% water changes daily with warm, treated, aged water and be sure there is enough algae for all the otos in your tank. If you aren't sure, it won't hurt to add in some raw or blanched veggies as a suppliment. Remember to remove anything not eaten within 2 days.
It is also better to have them in a breeding tank, or at least a tank with no other fish. Oto fry can be a tasty snack for anything that can catch them, and it isn't safe to have them in a community tank until they are at least an inch long.
You can keep Otos with a variety of other community fish, however anything that regards other fish as food (this includes Goldfish) are not a good idea as tankmates for otos. They are quick, but since they do not grow over 2inches long, many fish may decide they are tasty snacks.
I have kept mine with betta, snails, tetra, mollies, platies, guppies, yo-yo loaches, corydoras and so forth. Goldfish, cichlids, knife fish, and oscars are better off with other larger catfish. The easy rule of thumb is, if the fish is a carnivore or picks on other smaller fish, it's better off left out of your oto tank.