They are ok with bettas though it would be pushing it to have cories, do you have a hex or regular rectangle tank and what type of filter do you have. If you get the small species you might be ok, I would test your levels often for awhile though.
Cories need to be kept in groups of 3, and most get 2 inches long or more (there are a few pygmy varieties but they need to be kept in larger shoals). So, Cories would add up to 6 inches or more, which would mean you'd have at least 9 or so inches of fish. I think that would be pushing it too much in a 5 gallon. 2 Otos wouldn't overstock the tank, but you'd need to do 50% water changes each week or the nitrates would get too high and make the Otos sick (they are pretty sensitive fish). Also, I would not recommend getting Otos unless you have live plants in your tank. Check out this site if you would like more Oto info --> www.otocinclus.com
I was thinking of otos too but i don't have live plants and probably couldn't keep any because my tank doesn't have a light. So I should just leave it as a Betta only tank, or are their any other small catfish that could live with a Betta?
You need to have a light on your tank regardless of what fish you are keeping. Fish need 10-12 hours of light each day. Otherwise, the get a vitamin d deficiency and die. You should go pick up a hood for your tank--get one that's fluorescent, not incandescent. The incandescent lights aren't very good--they produce mostly heat and not light. A fluorescent hood may cost you $30 or so (US) but you need to have one or your Betta will probably die.
And I agree with ATM, just leave it a Betta only tank. What's your Betta's name?
The lid came with the tank but i did have to modify it to fit the heater and filter. It actually did come with an incandescent light but I got the tank from my cousin and the light never worked so I wouldn't be afraid to put a fluorescent light on it.
I would not put Cories in a 5 gallon with the betta, I do have 2 cories with my betta in a 5 gallon but I am changing the water 3 times a week and it still gets really dirty. I want to upgrade to a 10 gallon tank for them but am going to have to take a bit to do this. Also you never want to put more than 2 otos in a 5 gallon tank as they do not get enough to eat.
Pygmy cories can be kept in groups of 3, they like 6 but can survive quite well in groups of 3 and do well. Otos are the best choice for a 5 gallon tank though. They are tempermental for a first 30 day period but once they are through the first 30 days they can live a long and healthy and very hardy life. Many of us have otos who have become our oldest fish and are quite healthy and happy. I have 3 tanks with otos in them (2 in 2 of them and 1 in the 3rd) The one with the single oto in it is very nontypical as they like to be with others of their own kind but he shows no tendency for socialization and I needed to put him in a tank away from the others.
You can put the otos in a tank like you are describing even without real plants as long as you have some type of hiding places and an airstone running (or airwand they love those and will sleep on them). Plastic or silk plants are fine with them as well.
I guess it depends on the individual Pygmy Cories. I used to have 6 of them and they were always swimming around together. Now I'm down to 3 and they just hide all the time. Most good sources I've found say to keep them in larger shoals. But I guess some might be ok in a group of 3, idk.
If you don't have plants and want to get Otos then you'll need to monitor the nitrates very closely. They can get sick if the nitrates get too high. Plants help that a lot.