I brought a tetra home with its 5 gallon tank from a friends. The water and the tank were really dirty so I changed 80% of the water, cleaned the tank and changed out the gravel. Because the tetra looks fine after all that I decided to get 2 guppies today. After i put them in the tank, they have been swimming at surface of the water for the most part. One of them is more active than the other one. Is it normal for them to be swimming at the top? Thanks! PS I have filter so the water is getting cleaner slowly.