RO ice cubes is a bad idea, as they'll melt in minutes and lower your salinity.
I keep a 2L coke bottle of water in the freezer, it's at -18c. I had to use it the other day to bring my tank's temperature down, as it was nearing 27. It's usually at 26.6. With that in the tank, it dropped my 650L system by just .2 degrees and the big block of ice was warm water again, it took about 20 minutes.
So keep ice as a last resort but it's not a long term solution, as you wont' be able to freeze things fast enough to keep a sufficient supply coming.
The fan is a good idea, keep it blowing across whichever area gives as much surface area as possible, be it the tank's surface, or the sump's surface, but the display tank is usually larger.
If you've got any cover glasses or a hood on, removing it during the warm weather can help massively, obviously you'd have to do that with your fan anyway.
Invest in a temperature controller like the STC-1000, you build it into a project box and it's about £50 for all the parts. Once running, it'll cut the fans in when needed and so with the heater, so that they never reach a point where they're fighting each other, something that can happen with just a thermostatic heater and a fan.
Hope that helps, Mike.