I have a question about my green spotted puffers?

i have 2 gsps and one is bigger than the other. I have the in a 30 gallon tank with a divider because i have other fish. My questions are my smaller puffer has black on the sides of its belly that seem to not go away but its belly is white is that normal. Also wut kind of decor should i have in the tank. Right now i have one plant a live rock and 2 other types of tank decor i wanted to know what else i should put in there. My final question is how much should i feed them. ive been feeding them a half a cube of brine shrimp is that enough or to much? I guess just tell me everything i need to know to maintain a green spotted puffer