Question

the water level in an aquarium decreases slowly even though tank does not leak.what change of state is occuring?​

Answer 1

Answer:

It depends on how fast the level is decreasing. If it's fast then check again, you still might have a leak somewhere. If it is gradual and consistent then it's something else.

Well obviously an aquarium isn't a closed environment. Meaning air can get in and out.

Water from the aquarium evaporates and thus creates water vapour. That water vapour leaves the tank and over time your water level goes down.

Tropical warm water aquaria create more vapour then cold ones. Anyway there isn't really anything you can do about it.

Just add some water now and then. Or create a closed environment but that's a science experiment in itself. Cause the oxygen needs to come from somewhere and you don't want to suffocate your fish.