Stocking a 20 gallon tank

[fishwish34]

Hi,
I have a 20 gallon high aquarium. It seems to be cycled. When I last checked, the ammonia and nitrites were at 0 and the nitrates were at 10 ppm. The aquarium has been set up for almost a year.
For fish there are:
4 zebra danios (2 are long-finned, and are a bit larger than the other two).
2 male guppies
1 otocinclus catfish
All of these fish have been in the tank for at least 3 months.
How much more space do I have for other fish? Do any of my current fish need more friends of the same species? I like dwarf gouramis, honey gouramis, and platies, and am trying to figure out what else I could put in the tank. I realize a 20 gallon long aquarium would be better, but I didn't know that when I bought the tank.

 

[lauraf]

Hi, welcome.
A great website to check is AqAdvisor - you can enter the size and dimensions of your tank, what you have currently stocked, and what you would like to add. AqAdvisor - Intelligent Freshwater Tropical Fish Aquarium Stocking Calculator and Aquarium Tank/Filter Advisor
It will tell you whether you are overstocking or have fish compatibility issues.
Right now, your water certainly sounds like it is cycled. Any unexplained deaths or diseases so far?
We here can give suggestions about what would work in your tank though. What kind of filtration do you have?
For stocking advice from my point of view, assuming there aren't any major issues, here goes:
zebra danios would probably be happier being in a slightly larger school, like a minimum of six, long-finned will be fine with the short-finned;
2 male guppies are just fine as is;
otos like to be in a group, but you need to have some algae in the tank for them - if there is algae, add at least two more;
in a 20g mixing different gouramis is probably not going to work - choose between the dwarf or honey, add two of either, but not both;
platies are a great addition, if you add them do two females with one male;
depending on whether you can add more fish to this mix or not, I'd suggest something interesting that hangs out near the bottom. If AqAdvisor allows, maybe three+ dwarf cories?
Cheers,
Laura

 

[fishwish34]

My filter is an Aquaclear 30. It has a sponge, bag of carbon, and Biomax, which are these cylindrical pellets with little cracks.

 

[tom91970]

The AC 30 is a great filter. I use AC's on on at least four of my tanks. The carbon really isn't necessary.

I agree with Laura. Add a few more danios for a school, same for the otos. And you sure don't want to mix those two gouramies.

 

[fishwish34]

I think I will get 2 more danios, so there will be a school of 6. I'm not sure I have enough algae for more otos, though. Will the one be okay on his own? He doesn't come out much during the day, but I think it's because he's scared of me...I see him in the morning, sometimes, nibbling at the gravel.