one of my amano shrimp has had a black underbelly now as if its stomach is full for about 3 or 4 days, the other shrimp are all normal and clear and im getting a bit concerned
Are you sure they are not eggs?? Amano eggs are very very tiny. The eggs change colour at various stages of the pregnancy. Here's one of my pregnant amanos.
Not sure how many eggs but it looks like a thousand to me They won't breed in fresh water unfortunately . The newborns will need brackish to marine conditions to grow and after a period, back to fresh water. Here are a couple of links on breeding if it helps.