Again, platies have the same requirements as fancy guppies. However, platies enjoy water conditions with a high pH and high alklinity (hard water). Additionally, platies may become agressive nip the fins of the betta.
Mollies, as with all livebearers like guppies and platies, enjoy hard water with a high pH, the opposite of what a betta prefers. Additionally some mollies may become very aggressive if the tank is not big enough since mollies can get up to around 3 inches so a 5 gallon tank with 1 betta and 1 molly is pushing it. Mollies with shorter tails make better tankmates than the lyretail mollies since the fancy tails may cause the betta to mistake a molly as a rival.
Swordtails have the same cons as the other three livebearers, plus other characteristics that make them unapealing as tankmates. Swordtails live in fast moving waters while bettas live in slow moving rice paddies. Additionally, swordtail colors and tails may cause aggression from the betta.
Out of the picture
Fish listed here shouldn't even cross your mind, let alone your bettas path.
Not only are cichlids highly aggresive, but they live best in water conditions that are opposite than those of a betta. They enjoy high pH and hard water while bettas enjoy soft water with low pH. Additionally, most cichlids get way too big for tanks under 10 gallons. Most cichlids need aquariums 20+ gallons and that's when they aren't fully grown.
Many people think that putting a goldfish and a betta is a great idea since both are believed to be able to live in a bowl. Putting goldfish and bettas together is a horrible idea for many reasons.
- Goldfish are coldwater fish (enjoy the low 60's) and bettas are warmwater fish (enjoy the low 80's). That's a 20 degree difference. Try swimming in a heated pool at 80 degrees and then jumping into the ocean that's 60 degrees and you'll see how big a difference that is; quite a shock for both fishes.
- Goldfish are extremely messy so the minimal size tank necessary is 20 gallons for one goldfish. Since the average fishkeeper doesn't follow this rule and keeps their betta and goldfish(es) in a 10 gallon, nitrates go through the roof within a few days.
- Since goldfish are so messy, they require much stronger filtration than a regular fish. With a stronger filtration system, the stronger the current, and the more stressed the betta. A filter that strong would create a current that a betta cannot swim through.
Tiger barbs, or as I call them, fins shredders, are also an extremey bad choice for a tankmate. If you put a tiger barb with a betta, your garuanteed to get your betta's fins ripped into confetti by the next day or your money back. And becuase tiger barbs are so fast, the betta doesn't stand a chance to defend itself. Tiger barbs are basically a bad choice for a peacful community tank. They are better off living with cichlids who can fight back.
Gouramis (all species)
Gourami's make a bad choice for tankmates because they are in the same family as bettas. And in-laws never get along so don't expect them to be best buddies. Because they are in the same family, bettas and gourami's are more likely to fight each other. Additionally, both are very aggressive fish, adding to the danger of housing them together