Hello..
so i have a 12 gallon.
It currently has 2 serpae tetras, 2 black skirt tetras a yoyo loach.
They all seem happy.

Well,
i want to add some more fisH! The water readings are fine, and all are okay and play e.t.c.

What else could i add?
I was thinking neon tetras, zebra danios, dwarf gouramis, mollies, platies, cories and another yoyo or two (obviously not all listed) what would be a good combo? tah:fis h10:

Well, if you wants to add more gotta be more Sarpaes and Skirts they are fin nippers also schooling fishes so need at least 5 of each one ,so they are more than enough for a 12 gallon.

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55 Gals. Plants Anubias,Amazon Swords and more. Tetras: 6 Sarpaes.4 Red Eyes,5 Black Skirts,4 Giant Danios,2 Lemons, 2 Emerald Cory,4 German Blue.
10 gals. tank : Plants, Full of Guppies; 20 Gals. Tank planted Full of Guppies: 1 gal tank filled with Guppies

I agree, with schooling fish if you try to keep only two or three it is likely one or more of the following will happen:
1) They bully the other fish in the tank because they don't have enough of their own kind to interact with.
2) The strongest one picks on the weakest one until it is very stressed and usually dies.
3) They are very shy and and stressed, spending a lot of the time hiding.