Why is my platy fish sitting at the bottom of the aquarium?

Guppies like to be around 75F so it may be looking for a cool spot in the tank. Hot water rises so its warmer at the top than the bottom. Or there could be many other reasons. If you are doing everything by the rules below the fish should not be sick. :- 1 inch of fish needs a minimum of 1 gallon of water. :- Every tank needs a permanently running cycled filter. :- Every tank needs at least 50% of its water changed every week. Keep the above rules and your fish stand a chance. Fail in any of them and I can guarantee that you will have constant health problems with your fish.