Actually, if you have the tank planted well and only female bettas, you could have 4 in there.
I've experienced a female sorority in a 30 ltr, 5 infact and looked after the tank well with no hassle from the girls. If you are mixing with other creatures/fish, then no to a sorority.
We're just going to have to disagree on that one. I think you'd need to have kept numerous groups of girls for several years before responsibly advocating smaller tanks for sororities.
You should only keep one male full stop. 28 litres is also far too small for a sorority of females.
You could keep a male/female betta and a few very small, quiet tank mates but that all depends on the fish and the temperament of the betta. I personally wouldn't do it. I've got ADFs with my betta in a 28 litre tank but as they take up very different areas of the tank there are no problems.
Zero point having a filter if you need to turn it off. It won't stay cycled. Run a heavily planted tank if you don't want a filter or just put up with the noise. I've got three fish tanks in my bedroom (used to have 5 ) and trust me, you get used to the noise.
Thanks for the reply.
It is not me that wants to turn the tank off. It is my family and as I am living under their roof I have to live by their rules. I wish I could keep the filter on.
What temperature should bettas be kept at?
As I said, no point having a filter if it gets turned of for several hours a day. Just don't have a filter but do consider whether you should be keeping pets if your family insist that you can't do certain important things with them. If you go without a filter you'll need to either have a tank stuffed full of live plants to keep the ammonia down or you'll need to do water changes every day, which means you don't get time off for holidays and weekend breaks. Either way, you should get a liquid testing kit for ammonia, nitrite and nitrate. I'd also recommend pH has if your water has a pH over 8 I wouldn't get bettas as they are low pH fish.
Temperature wise - 26-28C. I use 27C.