You can also look into buying a chiller. I don't know a whole lot about them, but it might be worth looking into. My tanks get hot at 82 degrees, but I do a lot of water changes and that keeps the temps down.
Before you run out and spend some serious money on equipment like a chiller or AC in the room (although that might be nice for you too ), try the fan thing, seriously. Worst case scenario is you have an extra fan for around the house, or maybe you already have a fan you could try it with. It's an old school trick and I've seen it work to drop temps. Just don't be surprised if your water evaporates faster than usual (not a big deal anyway).
If you don't already have one add an air stone to the tank. At 90 degrees the oxegen level will be lower, the air stone will increase surface movement and increase O2 exchange. You could also drop the level an inch or 2 and add a spare HOB the falling water will have the same effect. The Rd should be fine for a few days.
Throw in some ice and then buy some ice packs. Surround the base of the tank with the ice packs. Now you want to put the ice in a plastic bag, to make sure the fish don't touch it (you don't want frozen fish )
Devils are tough fish, I know cichlids can tolerate mid 80's no problem. 90 would strike concern, turn on house AC, the tank will eventually cool but assuming for some reason you don't have AC, I would do several frozen gallon jugs of water to float in tank, add air stone/tube because at warmer temps oxygen deplets quicker. also your ammonia/nitrite levels have a tendency to spike higher/sooner at higher temps. Overall I think you'll be ok at 90 for short term.
...ice can be a very touchy way to cool down a tank..iv done it in emergencys but i don't recommend it ..
i have a cold water fish to begin with, i have a 40 tall and one of these fans keep my tank below 70...when the room it is in is around 80 or higher..just place it blowing down on to your tank, i lose about a gallon of water to evaporation a day.
Chillers work, but are really expensive. I battle heat issues all the time living in Orlando. Fans help alot. Im actually setting mine up now, via controller, to kick on when the tank reaches a certain temp. Try it before you spend a grand on a chiller. Just my advice of course
airstones as well.....they help on several levels with higher temp tanks