Cory catfish questions..

I recommend spotted cories or other dwarf cories in a group of at least 5. Seeing you have a large tank, you could even have 8 or ten. And maybe add some anubias attached to rocks or driftwood.

 

What is your ph and gh? I assume your tank is properly cycled. If you add more than 5 cories, I would add just half of them at a time so beneficial bacteria can build up.

 

Which species of cory? The larger species need to be in a shoal of at least 6 but the dwarf species (pygmy, habrosus and hastatus) are better with at least 10.

When you say other algae eaters, do you mean other than cories? Cories are not algae eaters, they are omnivores and need some meat in their diet. They are also not scavengers and need to be fed with a meat (or fish) containing sinking pellet.
Snails are good algae eaters and are fine with bettas. I have nerites in both my tanks. They will not eat every type of algae - nothing eats every type - and it is up to us to keep algae down to a minimum. All tanks will have some algae. If the tank has live plants, an imbalance between lighting, fertiliser and CO2 will cause excess algae. If there are no live plants, cutting down the time the lights are on might help.

 

Article all about cories kept with Betta

https://www.bettacarefishguide.com/cory-catfish-and-bettas-tank-mate-guide/

 

Well what are your water conditions recommendation for cories kept with a Betta

 

Maybe she should buy a 5 gallon tank just for the betta and and put the 20 gallon to better use by making it a community tank. That would make more sense. A betta doesnt need 20 gallons of space.

Walmart sells a 5 gallon glass tank by Aqua Culture that comes with a filter, hood, and led light for $30. Thats the one I bought for my Betta. You would just need to buy a small heater. My Betta is much happier in his own tank. I used to have him in a 10 gallon tank with 5 harlequin rasboras and he would hide a lot. Now he’s out in the open much more and even made a bubble nest.