Also what are the other water parameters at? Do you ever put small amounts of salt in your water? IF NOT, add 1 tablespoon for every 5 gallons of the large chunky aquarium salt, so 18 tablespoons, or 1 cup (dry measure) and two tablespoons in the tank. For the two heaters, 500W will heat a 90 gallon tank to about 9-10*F above the ambient temperature, which is fine IF the room the tank is in stays at 72*F(I am actually more surprised to hear that a 300W can keep a 180 at 88*F! Is your tank insulated?! I'd love to find out, my electric bill is killing me). They will not work as well if the temperature is closer to say, 65*F.