Posted: Mon Mar 02, 2015 12:37 am Re: How Large of Tank For Res
The Fluval will be just fine. In fact, for a tank that size, it will be more than enough.
The 90 gallon will do for now, but you will eventually need to separate the turtles when they grow up. In general, 1 inch of shell = 10 gallons of water. Female RES may get up to 12.5" (125 gallon tank), and males can get up to around 8" (80 gallon tank). This isn't always the case, but it's something you should definitely be ready for.
For now, though, your setup sounds fine. Be sure to provide the turtles with some kind of resting place near the water's surface.