[Keeping Rainbowfish not in groups]

12 minutes ago, James Black said:

Idk what kind of rainbowfish I have I'll send a picture 

 

That's a Melanotaenia fluviatilis or duboulayi. I would look up pictures of both and ID your rainbowfish that way. Either way if it is doing well, feeding, swimming around, etc. it will do fine alone. If in the future you upgrade to a 55 gallon or larger I would recommend adding in 5 more (with 3 males and 3 females) for the most vibrant coloration and activity.

[Keeping Rainbowfish not in groups]

5 minutes ago, James Black said:

its a 36 Gallon Aquarium

STOCKING:

Cardinal Tetras x10

Angelfish x2

Rainbowfish 

Guppies x2 (both males)

And then I have another angelfish, a swordtail and a bristlenose pleco in quarintine. 

I want to add a school of corydoras too

I'm using a sponge filter but will probably upgrade it either the tidal 35 or one of the aquaclear HOB.

I don't think I'll have enough space for 6 more rainbowfish

Do you know what rainbowfish species you have? I would agree you should add on a HOB or another filter and that you likely don't have enough space to have a school.

[Keeping Rainbowfish not in groups]

4 hours ago, James Black said:

Could I add a different species of rainbowfish, to keep him happy?

It's best to keep the same species together to prevent hybrids. Hybrids dilute bloodlines which is bad for the future of rainbowfish species. Try adding 5+ rainbowfish of the same species if you have room. Could you list your tank size, livestock, and filtration?

[Keeping Rainbowfish not in groups]

Rainbowfish are most comfortable in shoals of six or more generally of their own species; if they are intermixed each species should have it's own shoal. Certain species can interbreed with each other and form hybrids; this is not an infrequent occurrence.