Tank too large?

Same here, had one solo in a 29 gallon tank with cories. He bothered the cories sometimes when they were eating shrimp pellets (this particular betta's favorite food, by far...even more so than frozen bloodworms!) but never had any problem getting air or getting enough food...and a 29 gallon is a pretty tall tank.

If he's slow to eat, you may want to try some slow sinking crumbles or floating food in a ring feeder. Slow to dissolve foods like algae wafers always seemed to be a big hit with my bettas.

I'm not sure why people treat fancy goldfish and bettas as though they are handicapped fish that need shallow, small tanks with little flow. They are much more active when they have lots of space and a decent amount of current. I wouldn't point a Koralia 600 at a betta, but you get the idea ;)
 
i had one in a 29 and then into a 55 and i made sure he got a shrimp pellet in his favorite spot bc he wouldnt eat anything that was at the top of the tank only algae wafers and shrimp pellets
 
I have my orange betta in a 75 with my goldfish. It's actually set up more along the lines of a mbuna tank with driftwood and lots of rock piles and he loves it. Worst thing that's happened is some fin nipping.
 
I have my orange betta in a 75 with my goldfish.

bettas and goldfish require very different temperatures. bettas are tropical whereas goldfish are temperate/coldwater. your fish would be much more comfortable housed in different tanks.
 
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