grindalworm

I am currently using sponge as a substrate of grindal worm culture. But I learned there are various types of substrate used like coco peat , peat moss and so on. What do you use and what is merit of using your substrate? My current problem is that they smell very bad and water change is needed...

I feed grindal worm to my betta and guppy. And I feed cat’s artificial food to the grindal worm. There's no problem at all now, but I am very curious about what other people feed to their grindal worm. I will appreciate it if you would answer my question.