Can a RES turtle expert help me please?

Not an expert but do no a lot. They are fine in any depth of water. The real concern is keeping the water clean enough. I recommend the canister filter. personally like fluvals over other. the basking lamp should be placed over the area where it gets out of the water height depends on the temp put a thermometer there to make sure its not to hot. UVB lamp should also be no less than 12 inches away from the basking area. but the closer the better because the further away the less the turtle will receive. If they are that young the most common problem with them is fungal infections and vitamin deficiencies. Good food for hatchlings and proper lighting will normally take care of the vitamins but fungal infections can be caused by the habitat not being clean enough or it not getting the proper basking opportunities.