I recently flushed the rad in my 1990 Honda Civic wagon and replaced
all the coolant as well. I put a pellet of leak stop in there as well
as I seemed to be losing a small amount of coolant over time but could
not determine where the leak was from.

After flushing the rad, I've noticed
i) the heater on my car is not as powerful as it used to be
ii) the idle on the car, even after the engine has warmed up is around
1100-1200rpm.

Any ideas what may be causing this and how to remedy either of the
above problems?

Thanks again for all of your help. My previous problem with the gas
tank was that it was corroded in the neck and the expansion of the cold
fuel managed to break the tank. I've since replaced the tank and no
longer have a gas leak.