Is it possible for all freshwater fish to eat the same food?

I would say it's a mixed bag of yes and no. Ideally, you want to feed a balanced diet to all fish, containing at least a difference of 2-3 foods that are fed on a rotation. However, due to the size difference between bigger fish and the diet each species needs based on where it's mostly located, I would say more on the no size to make sure that you have the correct diet being fed to each fish (and that the food can fit within their mouth).

It's a tough one because as much as they would all attempt to eat the same food, you would need to look at all the reasons it could be an issue such as would they be able to eat it, would it be too big for them to eat. As much as it would be easier and cheaper to give all your fish the same food it may not be good for their diet and health to do that. I would definitely look into the fish you have and look to see what foods they can eat and what foods they can both eat and give them a variety of foods so that all fish get what they need and there is no risk of them going without or ending up ill because of what they ate.