Is this heater big enough?

[TJT]

Im quickly finding out that what a product says it can handle might not be totaly true. Anyways i bought a 90 galon tank and im using a Hydor Theo Submersible Heater-400 Watt, witch says its good for 80-110g tanks. Im just wondering if i should pick up another heater or if this in fact ok for my tank.

I also picked up a Eheim 2215 witch i belive is rated for up to 95 galons... and the more i read the more i realize ill need another filter for my tank. This is mainly why im second guessing the heaters "rating"

I usualy keep my water around 82-84 because of the Discus i have.

Thanks.