How many gallons is your aquarium 18x18x36?

1.33 gallonsAssuming that you mean 36cm x 14cm x 12cm, multiplying these together gives 6048 cm3. Converting this to dm3 (which is in other Words, 1 litre) by dividing by 1000 gives 6.048. Then, by dividing by 4.55 you get 1.3292307 etc. which rounds up to 1.33 to three significant figures.