Buy a Coleman tumbler or similar and place the bags inside before stuffing into your luggage. It serves a dual purpose since it holds the liquid inside the tumbler and doesn't leak even if the bags burst/leak (it is also hard, and will take alot of weight, which is good since the luggage is going to be stacked up).. And, they are also meant to keep the temperature constant since these are insulated.

Check in the luggage as you would normally. Then declare at Singapore customs as long as the limit is not over 30 fish or 3L of water.

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