What kind of turtle are you trying to keep first? Also you do know that 25 gallons of water will only work for a short period of time with the turtle till it needs a bigger amount of water. A rule of thumb for turtles on care and proper housing is per every inch is equal to 10 gallons of water. So the 25 gallons of water will last for only a short time till it needs more, then it will grow were the 55g will need to be replaced. You will need atleast a 75g tank to house a turtle, and that's even specific on what kind you get. Another thing to know, you won't be able to determine the sex till they get bigger, and with most species females get way bigger then males.
Another thing with your tank, your not going want to put gravel in it. Your either going want sand, or big rocks. With gravel the turtle will eat it, and trust me it may at times be able to pass it through, but times it won't. It can happen the first time r the millionth but taking that chance can be a deadly one and a costly one. With sand that passes through, with bigger rocks like river rocks they won't be able to fit in there mouths.
With the cork bark, through time it will get water in it and make it then useless, won't happen right away but through time it is going to, so you will be replacing it. Basking area needs to be comepletly dry when the turtles are on it. Your going need a heater, proper lighting (UVB and UVA), good filter, and good basking dock.
If you have any other questions ask, I own turtles and have for awhile so I can say I know my stuff about them.