How fast do the pellets sink? I don't think the tetras will feed off the bottom. Crushed up flakes for me seem slow enough for them to realize it is food and then they start hunting around. I feed Omega One flakes and have no issues
Have your tetras ever tried to eat the pellets in the past? Or has your betta always had first grabs? If you put 3-4 pellets at one time do any make it past your betta?
Try to release a few pellets with your fingers under water while you feed your betta on the opposite side of the tank as a distraction. If they don't take to the pellets then try to crush up flakes and release under water while distracting your betta.
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