Very new to planted tanks, what are ottos? I know it's and abreviation for some name of a fish and that everyone says that every tank should have one. What other fish and number of them would suit a 20gal long well?
Otto's are otocinclus wittatus. They will go well with any non predatory fish. They are good to use in a planted tank for cleaning purposes. They like to eat the algae and a nocturnal feeders. I have 4 in my 29 gal may get more.
Even though these guys are nocturnal doesnt mean they arent active during the day. They are great fish but fairly sensitive to traveling and being introduced into new tanks.
If your tank is planted I would look for fish that wont dig in your plants alot. But its up to you, what fish do you like? I suggest you go to your local fish store and look at several fish you like and watch their behavior, some are alot more interesting than others.
Ottos are fun to watch and great algae eaters.
Tetras look nice in planted tanks, especially small schools of them.
Alot of people with planted tanks use livebeares aswell, I'm not a big fan of them.
I have 5 in a 55g but when I upgrade to my 125g Im adding alot more. These guy's are adorable and Iv seen all 5 swimming around with my schoolers from time to time. Just be sure there Oto's and not something else
I would try and get at least 3 in your 20L but make sure their tummies are full for the most part and if not you will have to feed them since they won't be able to live off of algae alone if they are successful at eliminating it.
I think the most common O-cat is Otocinclus Affinis.
8-10 would be good could get more. I use the rule of 1 in. per gal., I don't always follow it though. I've got about 34 fish(maybe more) in my 29 gal. But if all you wanted was neons you could have around 20 even with a few otto and be alright. Just make sure you have a low ph 6.5 and soft water, neons can be alittle delicate. Otto's can be purchased all over they go for about $2 a piece