1.5 gallons or 15 gallons?
If the tank is only 1.5 gallons it is too small for any fish and you would be better off getting a 10 or 20 gallon plastic storage container. The small volume of water will get ammonia readings very quickly and cause problems to the fish.
A filter will help when it is established but that will take about 4-5 weeks before it has sufficient numbers of beneficial bacteria in it to keep the water clean.
Do you have a heater in the aquarium, or do you know what the water temperature is in the tank?
Tetras are tropical fish and depending on the species of tetra and the water temperature, that could also be a problem.
In small tanks you need to change most of the water every day and replace it with clean water that is free of chlorine/ chloramine. And you need to keep the feeding down to a bare minimum, no more than one feed every second day. If the water looks cloudy or smells, then do a 90% water change using clean dechlorinated water. If the fish are looking unwell or breathing heavily, or if they get a cream/ white film over their body or eyes, then do a 90% water change.
If the aquarium is only 1.5 gallons, you should take it back to the petshop and get a refund and then buy a bigger tank/ container for the fish. The smallest tank you want for any fish is 10 gallon (40 litres). The bigger the tank, the more water it has, and the better the fish do.
Aquariums should have a piece of polystyrene foam underneath them to help reduce any uneven areas on the table or stand they are on.
