I saw something today that may not be very good... The frog was going for a bloodworm, but so was Ruby. They both started trying to look menacing, with Ruby flaring and circling the frog and the frog waving its little arms around. They eventually backed off, but I'm kind of worried now.
Maybe you could catch ruby in the cup she came in and feed her in there while you feed the frog.
I had to do this with one of my female Bettas when I had a female betta tank. She would chase everyone during feeding so I'd catch her to give everyone a chance to eat. She was so smart. I'd put the cup in the water and she would swim in on her own, wiggling in an excited manner.
So anyway that's an option.
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Those lines are "stress lines" It's common in female bettas and like you said shows up when they are stressed. They also get vertical lines when they are ready to breed.
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