Hi Peter and welcome to the forum. Your English is just fine and I've moved your post.
Your 6.6gal will be OK for most Aphyosemion and if it has lots of plants plus a small sponge filter, 2 pairs or trios will be quite comfortable and don't forget floating plants (or a tank cover).
What species do you have in mind? Try the search button and if you can't find suitable answers, let us know.
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