Love watching Pacu swimming together, majestic when they become very big.
I have a pecan nut tree and toss in a handful of whole nuts still in hard shell. They love these floating treats. Become very excited, zooming up to snatch from below with a big swish of water. Followed by sharp cracking and deep grinding sounds which can be heard meters away. They grab, nibble, mouth, flip, rotate and reposition smaller pieces until it's just right for them. Often chased jealously by the ones who missed out. This is a food they really seem to enjoy. Nutritional value is good. Watching them playing happily is better.
Peanuts in shell would be similar or any high oil nut.