Assuming that your water condition is good, free of ammonia and nitrite and you used water conditioner during the change water, then your fish might be having gill flukes.(parasites)
When a fish is laying at the bottom, it means it's very sick.
At late stages of gill flukes infection, your fish will breathe very fast/gasping. Your fish will also show other symptom such as scratching against the tank or any object in the tank.
You have to quickly use some medication such as Praziquantel as gill flukes infection will kill the fish.
Although warm water can also cause fish gasping due to low level of oxygen, I think it only happens at very warm water such as above 86F(30C) and mostly in a crowded tank which rarely happen at home(more at fish stores).
The quickest way to treat if you don't have any medication at home, is as what Colin used to advise - 2 tablespoon of salt for 20 liter of water.
You can calculate your tank volume by this formula:
Volume = Length X Width X Height (cm) and divide by 1000.
Example:
If your tank dimension is 20 X 20 X 30 cm,
Volume = 20x20x30/1000
=12000/1000
=12 liter.
To convert gallon(US) to liter, simply multiply by 3.78.
10 gallons = 10x3.78
=37.8 liter (close to 40 liter).
Did you feed your fish with worms such as tubifex worms which often carry parasites with them?
