Is it safe to keep 5 guppies in a 15 litre tank?

The tank is a bit too small for that number of fish, particularly when you are dealing with livebearers- you're better off upgrading that tank & using your existing tank as a nesting tank to keep the babies safe from mum & dad until they mature.

They can multiply very quickly! A 2' tank would be a good start :)

Oh, and Guppies thrive in brackish water :)

That's about 7 gallons right there. Which you are correct on the fish being over stressed about the capacity. A minimum for at least four guppy's is at least 10 gallons fully cycled and heated. A 7 gallon aquarium would best fit a single betta fish with some shrimp or snail.

YES u can. Guppies are small fishes which can live in group also.

You can keep ur fishes in ur 15 litre tank but make sure it is filled with at leastt 10 litres of water.

I have some Guppies in a 10 L fish bowl and they are all fine.

But u shud also take care of filteration and do regular water changes as in small area ur water will b dirty very soon so u shud take care of ur filteration system also.

I would fill the tank up fully if the tank is only half full then how are you using a filter and heater ?

Frnd your aqurium is small. .Use atleast 10gallon tak. .Means nearly 40lt tank.. .

But I suggest to fill your tank at the top. . .