Housing guppies with bettas is done at the fish owner's risk.

Myth: Bettas can be kept with goldfish since both live happily in bowls.
Reality: This myth is multi-dimensional. Bettas are arguably unsuitable for keeping in bowls due to the difficulties in maintaining appropriate temperature. Goldfish should never be kept in bowls due to their waste output, oxygenation needs, and growth potential. Improper housing aside, the two species are incompatible. Bettas are small, solitary, aggressive tropical fish which require soft, acidic, still water and pristine conditions. Goldfish are large, social, fin-nipping coldwater fish which demand harder, slightly base, well-oxygenated and filtered water. They are also massive waste producers and hosts to many parasites. In short, the two species are utterly incompatible, and should not be housed together under any circumstances - especially in bowls!

UMBRA FishHotel Stackable Fish Hotel Aquarium Tank Betta Fish Unique Fun Housing

Oh the loud noises you will hear from the betta community speaking up against people that keep their betta fish in vases and bowls! If you are currently housing your betta fish in a vase or a betta bowl, for the safety of your eardrums, don’t go proclaiming it to anyone! You may have even been keeping betta fish in jars, bowls or vases for years and tell yourself that you’ve never run into a problem. So what’s all the fuss about then directed at these “cute” bowls and fashionable vases?

Is the Aqueon Betta Falls Aquarium adequate housing? : bettafish

UMBRA FishHotel Stackable Fish Hotel Aquarium Tank Betta Fish Unique Fun Housing Now I have an tolerant/aggressive betta fish and a betta fish that is VERY laid back and easy going. They are both males. What is your opinion won housing them in a 2.5 gallon tank.

Housing a Mystery Snail with my Betta : bettafish - Reddit

One of the most common questions regarding the health of bettas is along the lines of “why doesn’t my betta move around much?”. So very often the answer to that question involves turning the temperature up on a poorly heated aquarium. That question is also often asked by people that have their bettas housed in small fish bowls.

Betta fish are some of the most mistreated fish in the world

Myth: Male bettas only fight with other males; this aside, they are peaceful community fish.
Reality: Most male bettas will fight with anything that even remotely resembles another male in finnage or coloration. Some will attack any fish indiscriminately, regardless of its appearance. It is inherently risky to house bettas with other fish. Some bettas are too aggressive to be kept with any species, and many community species will damage the finnage of a betta. This species does best when kept solitarily due to its special environmental and social needs. However, community keeping is possible with careful monitoring and appropriate tankmates if the betta's personality permits. Communal housing should always be approached on a case by case, individual basis! Myth: Bettas are completely unsuitable community fish and can only be housed solitarily.
Reality: While the safest way to keep bettas is solitarily for both the betta's sake and that of its would-be tank mates, it would be inaccurate to suggest that bettas can never be housed with other fish. Placid males and females can often be housed in a well-planted community tank with mellow, dully-colored fish, as well as some aquatic invertebrates or amphibians. Careful monitoring is demanded, and the positives and negatives of the housing situation should be thoroughly evaluated prior to mixing species, but the community betta is not an impossibility.