You have an excellent environment for your betta! My bettas are also in 5 gallon, heated, filtered tanks.
Bettas do not need companions, and if yours is happy, I wouldn't change his environment. I would really suggest leaving things as they are.
I wouldn't suggest a mirror left at the corner of the tank. It can cause too much stress to the betta. I did this once for a picture, and the betta flared and swam erratically for 5 minutes. A happy betta swims with his fins held upright anyway, usually, so you don't need a mirror to make him "pretty".
For my very first pair of bettas, I did have them in a divided 5 gallon tank. It was too small for two bettas, but a good size for one betta. I wouldn't suggest dividing a 5 gallon tank. You'll most likely have flaring and posturing most of the time, which just leads to stress.
I believe that a 5 gallon tank is the right size for one betta, and to try to add other fish with the betta will cause overcrowding and stress, as the betta tries to defend his territory. Tetras are not a good idea; tetras can be nippy. I've seen a group of tetras gang up on and nip a betta, and this was in a 20 gallon tank. Also, tetras need to be in groups of at least 6,
IMO. One lone tetra with the betta will be very lonely.
Colorful fish in particular will attract the betta's attention - the betta may think it's another male betta and attack it.
Now that I've talked you out of another fish
I can suggest some possibilities. I have tried ghost shrimp with my betta in the 5 gallon tank. Search for "bettas and ghost shrimp" here; there are many posts on this topic. It seems to be a 50-50 chance that the betta will get along with (ignore!) the shrimp. Some bettas will eat the shrimp. My shrimp only lived one week. I don't think my betta killed it, but he was so curious that he wouldn't leave the shrimp alone at all. Just make sure you get a shrimp and not a prawn. Prawns have claws.
You could also try a snail. Apple snails get too big for a 5 gallon, but another snail such as an olive nerite snail would be a good size for a small tank. The hatchery I go to has this snail.
http://www.azgardens.com/pond_snails.php
You could also do a search for an
ADF and bettas. Some people say they are ok together and some have not had good luck with that pairing. ADFs have their own set of care requirements that you should reseach before considering one.
