I think best is pack into lugguage and cargo into flight.. My point of view is if u put live fish they might not let u check in and u might have problem boarding flight. Yes, ava allow some marine fish (check with ava on fish spices if it is control fishes) into singapore.
But thailand might not allow live fishes into the flight. So pack into lugguage and cargo in. No point put fragile, from my experiences they still throw. When u reach red channel is good is declare that u have live fish showing email from ava to them that it is allow under ava rule into singapore
my wife use to bring fishes in for me when she go for trip. She said that.