the BEST thing you could do would be to get only ONE Red ear to live in this tank. It's perfectly fine to put in a hatchling, they will enjoy it as much as an adult and do fine, but if you have multiple, when they become adults you will find them too crowded. In a 90 gallon, once you drop the water level enough to put in a basking spot (they only need a basking spot twice their own size, I would give them two spots like this with different temperatures so they can choose what they want at any time) you will be left with about 70 gallons of water. To get this to balance for easy filter maintenace and have enough space for exercise, you want about 1 inch of turtle per 10 gallons of water (or more water, bigger is always better). Planning on the adult size is the best bet. So your tank could hold 7 inches of turtle, or one male RES perfectly, or slightly crowded and slightly overloaded with one female RES.
Your basking spot can be as simple as a piece of cork bark wedged into the tank, or a stack of bricks with slate across the top, or it can be as elaborate as some folks here have done with plexiglass and such. The main point is that you have enough space for about twice their body size, that it is a spot they can get completely dried off on (it should drain so they don't sit in a puddle).
Filtration would be best done by a combo of a cannister filter to pull out junk and polish the water, and a UGF setup with powerheads like a normal fish tank (you can put large river stones over the thick layer of gravel on bottom to keep the turtles out of it if you wish). This combo provides a ton of bio filtration and the cannister sucks up gunk to keep it from clogging the gravel as quickly. It works like a dream and is as easy to maintain as I have ever seen any tank setup be.
viola... there you go!
and i'd like some input on the best setup. i'm not completely herpatologically (lol) illiterate. i have 11 reptiles already and i know about basking and uvb lighting. i just want to know about setups and land to water ratio and such.